Building a dungeon for my level 6 party to go through, and I have a pretty good idea of what the dungeon is, but am at a bit of a loss as to what should go in there.
The dungeon is the former lair of an ancient red dragon that was petrified by an event that happened in my setting a couple thousand years prior. Its mind is still intact (well, in the sense that it can still think, it has gone insane over that long time). Relatively recently, a Osyluth moved into the lair and has made a hobby out of probing the dragon's mind, searching for whatever info it can get out of it.
The devil's connection with its kind has been severed, so it wouldn't have any lesser devils working under it. The lair is a cave system just above the Underdark/Darklands, and the area above the lair is a tundra (wasn't when the dragon moved in, but a lot has happened in the area). Any ideas of what might be working for a bone devil under these circumstances?
An elite, somewhat singed elder Basilisk (level 6).
Not saying he stoned the big guy, but it'll get your players wondering.
Or two Basilisks for a rougher battle.
Or ancient Dracolisk?
Old traps, "high level insta-death" ones that have downgraded over time to the PCs' level, maybe going for inflicting conditions rather than straight damage. Maybe the trap even misses (like errant heightened Disintegration Beams) yet trigger a collapse or hazard the party actually has to deal with. Maybe it even blocks their only known exit!
Dragon pets kept in stasis, like maybe a Genie (in a bottle/mirror/etc.), whichever kind suits your storytelling/combat variety needs.
Good source of info if amicable.
A Black Pudding (7) the Osyluth lured into a room then Dim Doored out of. Since the devil can Dim Door, this may even be on a main path.
Most anything that digs, either cold-tundra ones digging down or aberrations/worms/elementals digging up.
If needed a new chasm could work for as a path for non-digging enemies.
Whoever/whatever informed the devil about the dragon, the informant likely driven made by the devil's torturing. A Derro Magister (5, but bump up as needed if fighting alone) could work or most any humanoid scholar.
Dragon victim(s) now turned undead, i.e. a Poltergeist (LE, 5) which couldn't really threaten the devil so may ally with it or at least enjoy how it tortures the dragon.
Sacrificed tribute to dragon?
Limited use spells from the dragon, but obviously downgraded and more like a level-appropriate hazard. Likely fire so that the dragon can't bother the devil.
Traversing over lava (which maybe attracted the dragon long ago). Maybe steamy nearby if also near an opening out into the tundra. Crossing lava you while blinded by steam should be scary since they'll assume enemies are afoot.
And maybe there are.
Winter Wolves (5, larger or smaller as needed) simply because I enjoy them and you've got a tundra nearby.
As for specific allies for the devil, whatever rounds out his "party" to give skills/abilities it doesn't have, though it's pretty capable as is.
Ones that inflict the enfeebled, frightened, or prone conditions (if alongside the devil) synergize well because of Sadistic Strike. Or maybe a grabber to attack PCs that can do Good damage.
A dragon-friendly battleground should also work well for them since they can Dim Door/Strike each round over all the tough terrain, giving time for its poison to do its work. Mixed w/ Invisibility, it can harass them quite a bit.
(Maybe give him a hiding healer!) Or a shape-changing duplicate? (Not sure what can copy a fiend's appearance.)
Anyway, have fun. Cheers.
I’m a big fan of using crevasses in situations like this... some aberrant horror can climb up out of it to fight the PCs as they pass over the crevasse. It’s a great excuse to get a monster in that doesn’t have to be explained away in terms of “why does the big bad have this in its lair?”
Also how about a group of tundra dwelling nomads who were lured in by the devil and now are part of a cult following it? Now you can reasonably do the whole bit with guards, patrols, a workshop/supply storage for loot etc.