Looking for some eldritch horror type inspiration for encounters.

[ToastyTobasco]

I'm looking to make an adventure branch in my upcoming homebrew campaign that delves underground to some ancient ruins beneath a city. No one knows about the ruins, only things that creep out on occasion. The underdark gives a wonderful slew of monsters to choose from and madness can have an interesting twist to it all.

I want my players to walk into this and become unsettled. I'm not looking for grimdark but I want that juicy otherworldly flavor in this branch. I plan for it to be the penultimate area to explore before the final act. I'm thinking of encounters like finding an enemy adventure party that has been down here too long and a fight ensues but as the PCs fight, their opponents match them point for point exactly for damage. *TWIST* There is a strange relic in the room and they are seeing illusions of themselves in the future and literally harming themselves.

Or: Doppelganger hunting the party and during a short rest, kidnaps and swaps places with the party member. (Pull the player to the side and explain whats going on temporarily.) Doppel tries to lead players to their doom. Replaced member could then either be found further in the dungeon or closeby but scuffed up.

Anything media wise that could be looked at for inspiration could help. Printed campaigns, your campaigns or ideas, books, anything! I want to give my players a real thrill ride before all hell breaks loose. Or they could unleash hell here, who knows?

[Kane0]

What level range?

[ToastyTobasco]

What level range?

I'm not 100% sure on levels and balancing that quite yet but I definitely want this one to be level 10 or beyond. Ideally I would like the campaign to end at 15 or higher but I'm not sure about that yet. I have most of the world and setting made, now it's filling the adventure slots and striking while the inspiration is hot again.

[Kane0]

Chuul are CR 4 and make excellent pairings with Aboleth.

Any number of failed illithid tadpole experiments (you get a mindwitness from illithid + beholder, what about ilithid + slaad, or vampire or troll or medusa)

Combine an ooze with an intellect devourer or energy-draining undead

A smart flesh golem with spellcasting and/or assimilation abilities

An invisible stalker operating a room full of gnomish contraptions and war machines

Redcap night-time attack

[ToastyTobasco]

Chuul are CR 4 and make excellent pairings with Aboleth.

Any number of failed illithid tadpole experiments (you get a mindwitness from illithid + beholder, what about ilithid + slaad, or vampire or troll or medusa)

Combine an ooze with an intellect devourer or energy-draining undead

A smart flesh golem with spellcasting and/or assimilation abilities

An invisible stalker operating a room full of gnomish contraptions and war machines

Redcap night-time attack

Ooooooh, very nice

[Ukala]

I’ve never actually read it, so I can’t give a first-person recommendation, but in doing research to DM Out of The Abyss, a lot of people recommended the book Veins of the Earth. It seems like it’s a sort of sourcebook for a lot of the kinds of creatures and encounters you’re looking for.

[Brookshw]

D&D sources:
Elder Evils (3.5e), some useful things, could steal environmental effects (e.g. sudden blooms, etc).
Gates of Firestorm Peak. Classic D&D adventure involving the Far Realm. There's a 5e update for it available on the DM Guild.
Lords of Madness (3.5e), full of aberration.
Have you looked at the short term madness stuff in the DMG? Do so.

Other games:
Deep Madness (https://www.kickstarter.com/projects/diemensiongames/deep-madness-second-printing), maybe check out some of the youtube content
Anything Call of Cthuhlu (dur).

Other considerations:
A lot of Lovecraft's works involve the stripping of civilization to expose something more primal. Work with that. Maybe even rage effects that potentially cause players to attack the nearest creature, friend of foe (will saves naturally).

Eldritch horror isn't just a list of things that belong on a sushi menu but instead involve beings from outside of time and space. So, screw with time/space/causality in the game. Have there be a cave behind a waterfall, the waterfall falls extremely slowly and can only be passed through by moving extremely slowly through it, otherwise it acts as a brick wall. Subjective or randomized gravity can also be a fun play thing. Add to the environment, e.g., dark environments, luminescent plants suddenly growing from rock faces, dimension door traps that suddenly move party members to another place (or another time, i.e., remove them from the game for a few rounds, though don't do that in combat).

Your players aren't going to be scared, so show them NPCs fears or mental states instead. Say another party came through here recently, find one of them huddled somewhere weeping and suffering from some madness condition. Find a journal of another party member, something like (brief version): day 6, Nordek told us he heard music, none of us hear it. day 9, Nordek fell behind, we had to backtrack for him. Found him standing there, said he was listening to the music. Day 14, Nordek's gone. Day 16, found Nordek dead, his bones shattered and his body molded into some kind of harp, his intestines the strings. Day 18, where's that music coming from?

[stoutstien]

Kuo-toa are always a good addition to this. Nothing like a grotto full of fish head humanoids that can cause their own nightmares to become reality.

Even better if somehow they are actually to cause of the existence of a the great old ones and elder gods.