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