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