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In (Re-)Running the Megadungeon, we looked at how you can evolve a megadungeon over time, actively playing it just like the players actively play their characters: You repopulate it. You modify it. You roleplay the inhabitants.

In the process, you create a dynamic experience that’s constantly surprising and delighting (and terrifying) your players, while also dramatically extending the amount of high-quality playing time you can get out of surprisingly simple prep.

Now I want to return to the series, flip things around, and take a closer look at how the players can evolve the megadungeon over time.

(If you’re here because you’ve innocently just started reading the series: Alert! The last link skipped you forward in time by a dozen years! Don’t worry! Any resulting temporal anomalies will resolve themselves shortly without disrupting your personal continuity.)

INTO CASTLE BLACKMOOR

Of course, almost any action the PCs take in a megadungeon will affect its future form. This is, after all, a back-and-forth dynamic. Killing all the lizardmen is what allows the elementalist to move in and set up shop, right? But what I want to spotlight today are the cases where the PCs more deliberately (and proactively) transform the dungeon.

Most of the examples we’ll be looking at here come from the open table campaign I ran in Castle Blackmoor, the original megadungeon created by Dave Arenson in which the modern roleplaying game was invented, and from which modern D&D was born. Running Castle Blackmoor provides a deeper look into how I set up and ran the campaign, but all you really need to know for now is that Castle Blackmoor sits atop a hill and beneath it lies the dungeon.

When the PCs enter the dungeon, this is the first room they encounter:

Speaking frankly and from experience, this room is incredible.

First, it’s too large for normal torchlight to fully illuminate it. So you’re immediately thrown into a fog of war.

Second, even if you have a more powerful light source, the shape of the room means that you can never see the entire room when you first enter it, no matter which entrance you use. Whether you’re entering the dungeon for the first time or returning to this chamber in the hopes of escaping to the daylight above, you can never be entirely certain if the room is empty… or if there’s something lurking just around the corner.

Third, and most importantly, there are ten doors. (Plus three more secret doors, including two hidden in the columns that aren’t indicated on the map here.) Literally the moment a PC steps into the Blackmoor dungeon, the player is confronted by the absolute necessity to make a choice: Which door are we going to open? Where is our adventure going to take us? The DM isn’t going to make that decision for you. You’re in control of what happens here.

Even if you have literally never played a roleplaying game before (and I’ve run Blackmoor for such players), this room inherently pushes them into actively engaging with the scenario while simultaneously teaching them that the game is about the choices they make.

The entire room screams player agency, and then holds forth the promise of endlessly varied adventure (every time you come back, you can pick another door). It tells you literally everything you need to know about Blackmoor, about dungeons, and about the game in an instant and without ever explicitly explaining any of it.

Playtest Tip: Describing the shape of this room verbally is impossible. If you’re playing in the theater of the mind, nevertheless make a rough sketch of its shape and be prepared to show it to the players. I kept a copy of the sketch I made clipped to my Blackmoor maps. But I digress.

The reason I bring it up here is that it’s a really simple example of player transformation of the dungeon: Confronted with all those doors, the players were confusing themselves when discussing their options and making their maps. So for the sake of clarity, they labeled the doors: First on the maps and then, shortly thereafter, in the dungeon itself.

Starting with the door to the left of the staircase, they labeled them alphabetically, A thru J. (Hilariously, however, the group who first did this missed one of the doors, so “J” ended up out of sequence.) The doors were first labeled in chalk (which one of the PCs had purchased), and this was later made more permanent when mischievous sprites in the dungeon started erasing the labels.

In doing this, my players were unwittingly echoing what Dave Arneson’s original players had done nearly fifty years earlier: After arbitrarily choosing the northwest door, they apparently fell into the habit of using it to mount most of their expeditions. It became known as the “Northwest Passage,” and eventually one of the players hung a wooden placard over the door with this name written upon it.

In my game, a later group took this even further, making coded markings at the various stairs in the dungeon to serve as navigational aids. These codes actually referred back to the door names (so for example, a staircase labeled G2 indicated that they were on the second level and this staircase would take them back to Door G… assuming that they hadn’t gotten lost or confused and encoded the wrong information).

TANGLEFUCK

Becoming lost and confused reminds me of another fun story from my Blackmoor table.

Looking at this section of the dungeon on the map, it seems fairly straightforward, although you may note Arneson’s devilish penchant for oddly angled diagonals.

One evening, however, a group headed into this section of the dungeon and began going in loops. Their map rapidly metastasized (because they were mapping the same corridors over and over again as if they were new passages), and by the time they realized what was happening they were hopelessly disoriented. They began making navigational marks (labeling walls and intersections), but because they were already lost, most of these marks were incorrect, contradicted each other, and only added to their confusion.

Fortunately, everyone at the table was having a great time with this, laughing uproariously whenever the PCs circled around, confident they were breaking new ground, only to come face-to-face with their writing on the wall or floor. (At this point, the sprites had already begun messing with the door labels, so there was also paranoia that something was here in the hallways with them and was altering their signs.)

One memorable moment came when they arrived at an intersection, confidently declared that they had gone left the last time they were here, so they were going to go right this time and they would definitely get out! … except that wasn’t right, and so they ended up looping back around and coming back to the same intersection again.

“Okay, so we definitely went left last time, so we need to go right this time.”

They did this four times!

Ironically, the door out was, in fact, immediately to the left of that intersection.

When they finally figured this out and, with great relief, headed through the door, one of the PCs stopped, took out some charcoal, and wrote in large dwarven runes on the wall, as a warning to all who might come here in the future: TANGLEFUCK.

And thus this section of the dungeon came to be known forever after.

OTHER TALES FROM THE TABLE

In another campaign I ran, the PCs began collapsing tunnels to prevent anyone else from entering a section of the dungeon haunted by a malevolent force. In yet another, the PCs memorably hired a group of mercenaries to guard the entrance to the dungeon and prevent other adventurers from entering. (An effort which met with mixed success.)

These player-led transformations are particularly wonderful in an open table: Because there are other players exploring the world who were not part of the group which made the original changes, those players get to discover them as artifacts of the world (and, conversely, leave their own transformations for others to discover in turn).

These long-term interactions across multiple sessions and groups can pay off in a multitude of gloriously unexpected ways. For example, I mentioned that my Blackmoor players began encoding navigational markers. But there were actually multiple characters who had the same idea, which meant that different groups were encoding information in different, overlapping ways. And then there was the memorable group where only one player had previously been part of an encoding group… and he screwed up the code. So not only did that group leave miscoded marks, but the other PCs in the group — who had been taught the incorrect method — carried that mistaken information into other groups and spread it even farther.

So who made these markings? Another group of explorers? Or monsters looking to trick the interlopers?

The fact that there are other real people interacting with this shared game world and that you can see the consequences of their actions and they can see the consequences of yours is intoxicating. (And can easily lead to motivating players to make even larger and more meaningful impacts on the game world.)

Even at a dedicated table, however, player-led transformations are great. It’s basically GMing on easy mode: You can often just lean back and take notes.

More importantly, the players are metaphorically throwing you a ball. They’re inviting you to play with them, and in the process making it a lot easier for you to generate your own responses that will continue to evolve the dungeon. (Like those sprites altering the navigational markings.)

All you need to do is pick up the ball and throw it back.

FURTHER READING
Treasure Maps & The Unknown: Goals in the Megadungeon
Keep on the Borderlands: Factions in the Dungeon
Xandering the Dungeon
Gamemastery 101

DISCUSSING
In the Shadow of the Spire – Session 33E: Maggots & Ratsbane

Someone threw themselves against the door Dominic and Tor were propping themselves against. It barely budged. They glanced at each and made a quick, unspoken decision. Dominic stepped away and Tor, timing things perfectly, yanked the door open at precisely the right moment.

A young elf woman – ebon-skinned like Shilukar – came stumbling through, thrown off-balance by the sudden disappearance of the door she had been planning to throw herself against.

Dominic and Tor were quick to take advantage – the former’s mace crushing her upper arm and Tor’s sword cutting deep into her thigh. She stumbled further down the hall, shouting over her shoulder. “Theral! There are six of them! Grealdan’s dead!”

Dominic looked through the open door and spotted Theral – the Brother of Venom that Tee had seen discovering Reggaloch’s body – beginning to cast a spell. He promptly slammed the door shut.

Back in Session 13B: The Tragedy at the Door, the PCs lost control of a doorway and nearly paid the ultimate price. In this session you can see them take control of a doorway and repeatedly use it to their advantage during the fight.

I often see doors getting ignored during fights. I think part of that is tied into some of the issues I discussed in Dungeon as a Theater of Operations: We have a tendency to get strongly locked into the idea that there’s a “fight keyed to Area 5” and, therefore, the fight takes place in Area 5 (and nowhere else). This, of course, frequently eliminates the door leading into Area 5 as being irrelevant.

I’m uncertain how much the proliferation of VTTs may be affecting this (since they often persistently feature the entire map of the dungeon), but “put the PCs in the room and then start the fight” is an attitude that you can even find infecting published adventures.

But just look at what a door can do for you! (Or to you.)

They’re natural chokepoints, allowing small groups (like PCs) to control their front line against much larger groups.

They can be used to control line of sight (and also effect), as seen here with Dominic using a readied action to slam the door shut and negate an enemy spellcaster’s entire action.

They can be used to separate groups, tactically isolating them and allowing them to be defeated in detail. (This is similar to attacking an enemy force when they’re halfway across the river.)

Conversely, if you move through a door and engage an enemy on the opposite side, then the doorway becomes your means of retreat. If you lose control of the doorway or are otherwise cut off from the doorway, then you’ll become trapped.

If someone is holding a door and using it against you, then you need to develop some method for breaching the door. (Or, alternatively, creating an alternative method of egress — using a window, teleporting, blasting a hole in the wall, turning ethereal, etc.)

In practice, of course, all of these myriad tactical considerations will be swirling around each other in the chaos of battle.

And we haven’t even started looking at doors with special features!

• How can a trapped door be used to your advantage during a fight? (Particularly if the trap resets whenever the door is shut.)
• What about doors that have magical effects attached to them? (Like magic portals!)
• You can get similar effects with non-magical portals, too! (For example, you might have a portcullis that’s rigged to come crashing down when someone pulls a lever.)

Some of these will create unique tactical opportunities. Others will simply complicate the ones we’ve already discussed!

Campaign Journal: Session 34A – Running the Campaign: A Confusion of Names
In the Shadow of the Spire: Index

DISCUSSING
In the Shadow of the Spire – Session 33D: The Hell Hound at the Door

At the far end of the room there was a small wooden bookshelf containing a dozen assorted volumes. These, of course, caught the particular attention Ranthir, who was also delighted to discover that one of them was a thick tome of spells.

Ranthir was not able to study the spellbook completely, but the illustrations of eyes being burnt away with acid were enough to leave him concerned.

NPC spellbooks are a huge pain in the ass to prep.

I’ve previously shared a tip for improvising NPC spell lists, but when it comes to their spellbooks, there is no shuffling: You mostly just need to commit yourself to the rote task of writing down lists of spells for each level.

I guess there is one potential cheat: Just replicate the caster’s prepared spell list and call it “good enough” for the spellbook, but I generally try to include at least two or three “extra” spells per level. (Partly for the sake of verisimilitude. Partly because it gives me some stuff to play with if the spellcaster should become a recurring characters. See, also, The Principles of RPG Villainy.)

I heartily recommend prepping these spellbooks as handouts. You don’t have to get fancy or anything: But a prepared piece of paper with all the spells listed on it that you can hand to the player at the table is just a great way of making the spellbook a concrete and persistent resource.

Way back in Session 3, we actually saw Ranthir loot Collus’ spellbook:

COLLUS’ SPELLBOOK

0 – acid splash, arcane mark, dancing lights, daze, detect magic, detect poison, disrupt undead, flare, ghost sound, light, mage hand, mending, message, open/close, prestidigitation, ray of frost, read magic, resistance, touch of fatigue

1^st – animate rope, burning hands, comprehend languages, detect undead, feather fall, floating disk, shield

For a low-level wizard, this was a treasure trove of new spells. Deciphering and copying this spellbook became a significant, long-term project for Ranthir. In every spare moment he could afford— and Ptolus certainly makes it difficult to find a spare moment! — Ranthir bunkered down and studied. You can find references to this in Session 7 of the campaign journal, but it was actually happening quite regularly:

Dominic expended himself in channeling the holy energy of Athor to heal as many of their wounds as he could. Elestra’s battered body was restored entirely and Ranthir was left with only a weariness from the blood he had lost. But the painful wounds to either side of Agnarr’s neck refused to close and, after inspecting them, Dominic concluded that Agnarr would need a full day of rest under his ministrations.

So Dominic settled into Agnarr’s room, praying occasionally and generally tending to his wounds. Ranthir retired to his own bed and set to work copying an additional spell from Collus’ spellbook into his own that would allow him to detect the presence of the undead – he wasn’t sure why, but he had a sneaking suspicion it might come in handy.

Magic being as powerful and versatile as it is, of course, the power unlocked by virtue of an NPC’s spellbook can often be the greatest and most valuable of treasures. In Session 12, for example, there was a direct pay-off for all of Ranthir’s hard work:

Tee led them to the river’s edge and then glowered down at it. Seeing the noxious water again – the edges of the cavern floor corroded and blackened where it met the river – did nothing to distill her fears. She had no interest in trying to wade these waters, no matter how calm the current might be.

Ranthir, however, was able to tentatively offer a possible solution. He had never stopped using the few spare moments in his day to study the spellbook they had wrested from the body of Collus (Toridan Cran’s arcanist), and one of the spells he had deciphered from its contents would allow him to conjure forth a floating disc of pure energy. It was a small disc and would only carry one of them at a time – but it should be a relatively trivial matter for him to ferry them across the river and, when the time came, to ferry them back again.

With the spellbook as a tangible prop (tucked away with the PC’s character sheet) and its study as a project, an NPC’s spellbook becomes a lovely and persistent reminder of everything that the PCs have accomplished — their history made manifest in the present.

I also love to use NPC spellbooks as a delivery mechanism for non-core spells. This might be a spell of my own creation, but more often than not I’m just culling awesome stuff from a variety of supplements. Perusing Vocaetun’s spellbook, for example, gave Ranthir access to a couple new spells:

VOCAETUN’S SPELLBOOK

0—acid splash, arcane mark, assess creature, dancing lights, daze, detect magic, detect poison, disrupt undead, flare, ghost sound, light, mage hand, mending, message, open/close, ray of frost, read magic, resistance, touch of fatigue

1st—acidic curse, color spray, comprehend languages, endure elements, protection from chaos, protection from law, ray of enfeeblement, reduce person, shield, silent image

2nd—invisibility, minor image, mirror image, obscure object

3rd—displacement, fly.

Assess Creature
Divination
Level: Brd 1, Clr 0, Drd 0, Rng 1, Pal 1, Sor/Wiz 0
Components: S
Casting Time: One standard action
Range: Close (25 feet + 5 feet/two levels)
Target: One creature
Duration: Instantaneous
Saving Throw: Will negates
Spell Resistance: Yes

With a wave of your hand, you determine the Hit Dice of one creature. This spell is foiled by any type of magical disguise, polymorph or shapechange.

Acidic Curse
Evocation [Acid]
Level: Sor/Wiz1
Components: V, S, M
Casting Time: 1 standard action
Range: Close (25 feet + 5 feet/two levels)
Target: One creature with eyes
Duration: Instantaneous
Saving Throw: Fortitude negates
Spell Resistance: Yes

You cause a victim’s eyes to fill with acid, inflicting 1d6 points of acid damage and blinding the target for 1d4 rounds. Creatures who suffer no damage from the acid (due to a successful saving throw, an immunity, or a spell granting resistance) are not blind.

Material Component: A bit of ragweed.

Open Game License

Using loot as a vector for delivering original spell content was once quite common in the earliest days of the game, when the core rulebooks featured only a paucity of spells that were “commonly” known. Then, as now, when your players learn the benefits of getting their claws on a spellbook, an encounter with an NPC throwing around strange eldritch arts becomes not only terrifying, but also exciting — redolent with the promise that they’ll soon have the opportunity to loot and learn those arts for themselves.

Campaign Journal: Session 33E – Running the Campaign: Battles at the Door
In the Shadow of the Spire: Index

SESSION 33D: THE HELL HOUND AT THE DOOR

December 28th, 2008
The 18th Day of Kadal in the 790th Year of the Seyrunian Dynasty

They headed out the opposite door and found themselves in another hall with doors to both their left and right. Arbitrarily choosing the door to the left, Tee picked the lock. Agnarr kicked it open.

The room beyond was extremely untidy, with a fetid smell that seemed to peremeate everything. The simple furnishings were crude and ill-kept. A weasel-faced man lying on the far bed jerked awake as the door crashed open.

Agnarr hesitated for a moment, uncertain whether the man might be a prisoner or not (the door, after all, had been locked and the room stank). But then he noticed that there was a sword laying on the bed and the man had a dragon pistol strapped to his side.

Agnarr charged with Tee on his heels. They hoped to silence the man before he could say anything, but he dove adroitly off the bed and rolled to his feet, firing his dragon pistol. He started shouting for help.

Tor and Dominic, still in the hall, turned and headed for the door at the far end of the hall, throwing their weight against it.

Tee, meanwhile, circled to the side of the weasel-faced man. Her blade darted here and there, keeping the man’s blade completely engaged while Agnarr came up from the other side and delivered the killing blow.

Someone threw themselves against the door Dominic and Tor were propping themselves against. It barely budged. They glanced at each and made a quick, unspoken decision. Dominic stepped away and Tor, timing things perfectly, yanked the door open at precisely the right moment.

A young elf woman – ebon-skinned like Shilukar – came stumbling through, thrown off-balance by the sudden disappearance of the door she had been planning to throw herself against.

Dominic and Tor were quick to take advantage – the former’s mace crushing her upper arm and Tor’s sword cutting deep into her thigh. She stumbled further down the hall, shouting over her shoulder. “Theral! There are six of them! Grealdan’s dead!”

Dominic looked through the open door and spotted Theral – the Brother of Venom that Tee had seen discovering Reggaloch’s body – beginning to cast a spell. He promptly slammed the door shut.

Almost simultaneously, with a powerful sweep of his sword, Tor caught the dark elf woman in the side of the face – his sword cut through one cheek, passed through her mouth, and out the other side. Her severed jaw fell to the floor and her body followed after it.

They took a moment to collect themselves and then threw open the door again.

HELL HOUND AT THE DOOR

A hell hound was at the door!

Like the ones that had attacked them at Pythoness House, the hound’s skin was cooled lava and its gaping mouth was a lake of fire that gouted a cone of flame down the length of the hall.

Beyond the hound was a massive chamber, its walls painted in horrific combinations of kaleidoscopic color. On the far side of the hall they could see a set of wide stairs leading down. To one side of the room stood Theral. At first there appeared to be six others on the other side of the room, but then they realized that there was only one man there – Vocaetun, the cultist with an ebon hand tattooed on the front of his neck – his form blurred and duplicated a half dozen times.

Tor and Agnarr squared off against the hell hound, rapidly reducing it to a pile of slag-like magma.

Theral, seeing the body of the dark elf woman and watching the fighters demolishing his hound, cursed and then shouted to Vocaetun. “Hold them here while I fetch the damn rats!”

Theral ran off down a side corridor. Vocaetun glared at his retreating back.

As the hell hound finally collapsed, Vocaetun waved a wand in Tor’s direction and then disappeared. Tor felt his eyes burning as they filled with acid.

Elestra leaped over the magma pile and headed towards the hall that Theral had dashed down. Rounding the corner she skidded to a stop.

“There’s a wall!”

“Don’t believe it!” Tee shouted, remembering the illusionary wall that Uranik had spoken of.

But Elestra hesitated. She didn’t want to throw herself into the unknown without the others to back her up.

Then Tee screamed.

Vocaetun had reappeared and hit her with an acidic curse that turned her own tears to caustic acid, having somehow circled around in the meditative chamber behind them. But none of them could see that – not even Tee, who was now clawing at her burning eyes.

Ranthir, seeing Tee’s reaction, knew that it must have come from behind them. “Over here! Tee’s being attacked!”

The others closed in on Vocaetun, but between the mirrored images dancing around his figure, the blurring displacement that seemed to cheat their vision, the blinding attacks from his wand, and the tight quarters, things quickly got confused and cramped.

Vocaetun mounted a fighting retreat back across the meditative chambers into the hall on the far side and then through a secret door into the kaleidoscopic hall. He had been hurt and was clearly beginning to panic. Once he was through the secret door, he broke into a pell-mell run across the hall – heading towards the stairs on the far side.

Running the Campaign: NPC Spellbooks – Campaign Journal: Session 33E
In the Shadow of the Spire: Index