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