From “The Tale of Satampra Zeiros” by Clark Ashton Smith:

The basin, I have said, was very large; indeed, it was no less than six feet in diameter from the floor. The three legs that bore it were curved and massive and terminated in feline paws displaying their talons. When we approached and peered over the brim, we saw that the bowl was filled with a sort of viscous and semi-liquescent substance, quite opaque and of a sooty color. It was from this that the odor came — an odor which, though unsurpassably foul, was nevertheless not an odor of putrefaction, but resembled rather the smell of some vile and unclean creature of the marshes. The odor was almost beyond endurance, and we were about to turn away when we perceived a slight ebullition of the surface, as if the sooty liquid were being agitated from within by some submerged animal or other entity. This ebullition increased rapidly, the center swelled as if with the action of some powerful yeast, and we watched in utter horror while an uncouth amorphous head and dull and bulging eyes arose gradually on an ever-lengthening neck, and stared us in the face with primordial malignity. Then two arms — if one could call them arms — likewise arose inch by inch, and we saw the thing was not, as we had thought, a creature immersed in the liquid, but that the liquid itself had put forth this hideous neck and head, and was now forming these damnable arms, that groped toward us with tentacle-like appendages in lieu of claws or hands!

I’m going to have to start using more water elementals!

If you wanted to get fancy, you could certainly pimp out the stats of a large water elemental to represent some of the creature’s other abilities from the story (knicking the stench ability from the troglodyte as we go):

Speed: 40 ft., swim 90 ft.

Frightful Presence (Ex): Characters with less than 8 HD who perceive the malignant shape-shifting of the elemental water terror must succeed at a Will save (DC 14) or become either panicked (50%) or paralyzed with fear (50% chance) for 2d4 rounds. Even characters who succeed on the saving throw are shaken, but those who succeed on their saving throw are immune to the creature’s frightful presence for the next 24 hours.

Stench (Ex): The sooty admixture of the elemental water terror’s primordial form exudes an unsurpassably foul odor. All living creatures within 30 feet of the elemental water terror’s must succeed on a Fortitude save (DC 18) or be sickened for 10 rounds. Creatures that successfully save cannot be affected by the same elemental water terror’s stench for 24 hours. A delay poison or neutralize poison spell removes the effect from the sickened creature. Creatures with immunity to poison are unaffected, and creatures resistant to poison receive their normal bonus on their saving throws.

But even this isn’t really necessary: All you need is a water elemental’s stat-block and that beautifully lurid description and you’ll have an encounter far more terrifying than that provided by any ordinary water elemental..

(Tip for adapting the description: Insisting that the characters stand stock still for more than 6 seconds watching the slow, inexorable emergence of their doom is, quite rightfully, frustrating to the players. You’re taking control of their characters away. But if you simply prelude with “time seems to slow for a long moment as” then you can achieve the same effect without taking control of the PCs away from the players.)

I also recommend “The Tale of Satampra Zeiros” quite highly. It reads like a prototypical D&D adventure with a couple of thieves for the main characters and is a little like reading a mash-up of Lord Dunsany’s lyricism and Robert E. Howard’s primitive adventurism. It can be found in The End of the Story, which is Volume 1 of The Collected Fantasies of Clark Ashton Smith. Smith deserves a place alongside Lovecraft and Howard, but is oft forgotten. Although he is not listed in AD&D’s Appendix N of recommened reading, Smith’s influence feels immense.

Honeytrap Lycanthropic Templates Untested: Fungal Traps