Here is the full list of all 12 Ufflegrim achievements.
Complete Spine's Peak.
Complete a floor on Daemon Lord difficulty.
Complete Floor 100.
Complete Floor 100 on Hardcore mode.
Complete Floor +100.
Purchase the big yellow skull skin.
Obtain 100 cards in your collection.
Defeat the Corpse King and his minions.
Defeat the angry cashier.
Encounter rainbows.
Reach 500 kills in Crafted Dungeon survival mode.
Assemble all relic pieces.