Lost relic in the mountains of Fneb: just the battle
A super-squad of do-gooders team up to carry a holy relic (tuning forks) into the cavern and use them to collapse the cavern.
Character aspects up for grabs that somebody can claim during the game
* Secret betrayal of the party; maybe because of corruption by the fiend, maybe a ransomed loved one, maybe just selfishness… make up a good story
* Sole survivor lives to tell the tale
* Noble sacrifice -- somebody needs to stay behind to make sure the relic works, or needs to pull down the chasm on top, or in some other way take one for the team.
* Grudge!  Anybody that comes up with a good back story on how this cleric did them wrong on the past gets bonuses.
The tuning forks
These are what the players will use to collapse the cave.
The heroes should smash some jars and some glowing lights will fly out and then cluster where the tuning forks should be placed.
Track ghouls to the caves
Locals say how once a month at night, ghouls raid their village and dig up corpses from their graves and then drag them away.
If they can't find any graves, they attack the farms.
Ghouls aren't very smart, so it should be easy to follow them back after a raid.  But the party can't allow any more innocent folk to be murdered as bait.  Instead, they have potions they can pour on straw scarecrows that will transform the scarecrows into real corpses (hopefully).
The party should leave the corpses out on tables in the graveyard and wait for the ghouls to arrive after it gets late.
The ghouls are actually a bunch of different undead… A smart leader, a few soldiers, and a bunch of zombies dragging a wagon.
Room 1: outer cave
Is a natural cave, entrance hidden from the outside.
Inside, walls covered in puffball mushrooms that will release toxic spores if disturbed, like maybe by being thrown into them
In the back of the cave, around the curve, is an iron door, locked.
In the curve, lots of bones and rot are under stalagmites.  Powerful stench of recently rotted meat.  And lots of flies.
Do INT check and predict ropers might be hiding in stalagmites.
There are several magic eyes in the room as well.  Detecting magic will reveal them.
The ghouls toss a corpse to the ropers to distract them while they go by.
Room 2: carved tunnel stairs down to cavern
Door opens to steep stairs down the tunnel, carved into rock.
Walls etched with writing, which are really glyphs of madness and terror.  Reading the glyphs triggers hallucinations of armies of zombie slaves.  Failed saving throws lead to mean incoherent babbling and screaming and panic and a desperate desire to flee the tunnel for the surface.
After a few hundred steps, there is a tee.
On one side, it leads to a gate, a big cavern where prisoners sit.  This is where the ghouls keep those they capture.
On the other side, stairs turn and in another angle, more steeply down into a tunnel.
Tunnel leads to a big door in a wall.  The wall and the door are covered in runes and glyphs.
This door is magically barred and rigged with magic traps.
Room 3: huge cavern with ponds
The tunnel was carved in order to get to this gigantic hollow cavern deep under the mountain.
The ceiling is very, very high.  The floor is wet,  and walls have holes in them.  An underground fills half the room.
Zombies are chipping away at the sides of the walls.  There is a huge rock pile.  
Room 4: site of reanimation / awakening / portal magic
The rock has been carved to expose a gem inside.
A giant eye floats inside the gem.
The rock.
The cleric will try to rush to free / wake up the old one.
Anyone approaching the relic needs to resist a charm attack.  Every turn gives another chance to snap out of it, and once snapping out, that character is immune from subsequent attacks.
Necromancer / cleric tactics
Use blink to jump to the ethereal plane.
Summon giant leeches and other monsters from the walls of the cave.
Surround the platform with walls of fire or spinning blades.
Resurrect corpses to finish work.
Rather than slay the party, he wants them to join him.