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STEALING THE JEWEL?

As written, if the PCs have the Jewel of Three Prayers and the Rivals are Unfriendly towards them, then the Rivals will attempt to steal the Jewel. This is also listed above as one of the general courses of action that the Rivals might pursue.

I would be extremely cautious about having the Rivals choose this course of action. I’ve been DMing for awhile and, in my experience, there are two Unforgivable Sins that an NPC can commit:

1. They can kill a PC’s pet.
2. They can steal the PC’s shit.

Anything else is probably negotiable, but these are almost always points of no return.

So if the Rivals steal the Jewel? Particularly early in the campaign before the PCs have established a relationship with them?

The Rivals are dead meat.

I’m not saying you should never do this. If it makes sense, then roleplay truthfully.

I’m just saying that you should be prepared for the consequences, which could very easily see the Rival’s role in the campaign come to an abrupt (and messy) end.

THE GMPC PROBLEM

If the Rivals are friendly with the PCs, it can quite logically end up with them joining the PCs so that they can all work together. As I mentioned above, Call of the Netherdeep actually scripts exactly this moment at the very beginning of the adventure:

If the characters are on friendly terms with the rivals, the rivals meet up with them soon after the characters’ breakfast with Elder Ushru.

Ayo Jabe doesn’t mince words; she wants to know what they found in the grotto. If she gets the sense that the characters have stumbled onto something big, her eyes grow wide. She decides that she and her group want a piece of the action and proposes that they travel with the characters, saying that there’s safety in numbers. A character who makes a successful DC 13 Wisdom (Insight) check realizes that she isn’t hiding anything and wants nothing more than to be a part of a grand adventure.

Call of the Netherdeep quietly assumes that the PCs will turn this offer down, but it seems far more likely that the PCs will agree with Ayo Jabe’s logic…

… and now the GM has to deal with five GMPCs.

Honestly, this feels like a huge headache to me.

GMPCs are not the same thing as NPCs. A GMPC is a GM-controlled character who is functionally the same as a PC in the adventure: they’re an equal member of the party and you could basically imagine an invisible player at the table controlling them as such.

It is possible to have success with such characters, but it’s far more common for them to fall into one of two pitfalls:

First, the GMPC can hog the spotlight and/or be used to railroad the players. This may be because the GM wants to do this (bad GM, no cookie), but it’s often not intentional. The core problem here is that the GM has privileged information (i.e., everything in their notes). During prep, they can predict exactly what the GMPC will do, and this can become a seductive crutch for them to fall back on. During play, their knowledge of the scenario inherently biases their decision-making. And even if the GM erects an impeccable firewall around the GMPC, the other players know that the GMPC has this privileged information and it will affect their relationship with the GMPC and the GMPC’s opinions.

(Imagine that you had a player at the table who had read the entire adventure and that the other players knew had read the adventure.)

Second, the GMPC can become a weird kind of half-character who awkwardly doesn’t participate in group decisions and/or frequently “vanishes” from the game world because everyone forgets that they’re there. (This can even happen because the GM is trying to avoid the first problem: Knowing that the players will privilege the GMPC’s opinions, for example, they just never have the GMPC offer an opinion.)

So even running one NPC companion effectively can be a big challenge. Five GMPCs at the same time? That probably doesn’t just quintuple the difficulty; it’s almost certainly exponential increase in difficulty. Even laying aside the inherent difficulties, juggling those five characters and making sure they are consistently a living part of the campaign world is going to chew up a lot of your mental bandwidth. There’s also combat to consider: all those GMPC turns are going to slow combat down.

Speaking of combat: All those extra GMPCs are going to have a big impact on the balance of combat encounters. And, importantly, the adventure isn’t designed for this. Running 5^th Edition for a group of 10 PCs is infamously difficult, but Call of the Netherdeep seems to just blithely assume that it will make absolutely no difference at all.

If you’re comfortable trying to run five GMPCs, go for it.

For everyone else, I’m not saying you should never allow the PCs and Rivals to team up. But I could certainly take efforts to make sure that this is only a momentary state of affairs.

Redirect the Rivals into supporting action off-screen. In other words, the PCs do X while the Rivals take care of Y. This is a little difficult in Call of the Netherdeep because of the linear design of the campaign, but it can be managed. For example, they might go to research the Jewel of Three Prayers somewhere else and then join the PCs in Bazzoxan. In Ank’Harel, they might volunteer to infiltrate an enemy faction. And so forth.

Encourage splitting the party, with each smaller group having a mix of Rivals and PCs. (For these scenes, you might consider letting the players whose PCs are not present take on the roles of the Rivals, particularly for combat.)

Remember to debate the agenda. Our methodology for running the Rivals (i.e., they should frequently believe that the group should be pursuing a different goal or, if they share a goal, that there is a better way to achieve it) will naturally lend itself to either splitting the party or breaking the alliance between Rivals and PCs entirely. Don’t be afraid to lean into this, as the aftermath will heighten the tension between the groups to delightful heights.

CONCLUSION

Ultimately, this all boils down to a simple formula:

1. Roleplay truthfully. (Actively play the Rivals and track the relationship gauge.)
2. Debate the agenda. (And force the players to think about and defend their choices and opinions.)

But this formula will manifest itself with an infinite variety at the gaming table, as the Rivals and PCs collide spectacularly in myriad ways. The unpredictable nature of these conflicts itself will bring the drama — and the characters — to vivid life. As you choose to actively play with them, the players will feel the fundamental reality — the ineffable uniqueness — of the events happening at your gaming table, and they will rise to the occasion.

FURTHER READING
Call of the Netherdeep: Running Betrayers’ Rise