Here is the full list of all 20 Tenebrous Dungeon achievements.
Burn to death on a campfire
Use a teleporter for the first time
Eliminate all enemies on any floor
Discover 4 secrets on any floor
Destroy 20 barrels on any floor
Permanently increase maximum health 5 times
Permanently increase ranged skill 5 times
Permanently increase melee skill 5 times
Destroy 30 headstones in the Graveyard
Defeat Captain Stauner
Defeat Sir Clifton Cake
Defeat the demon warlord Dysnomiak
Defeat Sir Phyreheart/Moort
Defeat Chief Sentinel Malorik
Achieve Level 8
Score 100,000 points
Defeat 'King' Bartholomew Black
Beat the game on 'Invader' difficulty
Beat the game on 'Knight' difficulty
Beat the game on 'Warlord' difficulty