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