Here is the full list of all 19 Catacombs of the Undercity achievements.
Find the Ultimate Ending in any mode
Find the ultimate ending in Classic Mode
Play three games of Bladefinger
Help Kuldrum kill Ratlings
Find the best armour
Make it past a training course
Gain entry to the Red Hand Guild
Fall from a bridge
Reach Undercity
Find the best weapon
Blind a Shark Beast
Contract two fateful diseases
Release some prisoners
Escape from the plague carrier maze
Join the Red Hand Gang
Hire all mercenaries available in Undercity
Return Mortphilla's restorative
Gain respect from the Black Hulk Mob
Return a holy relic