The Mad Moxxi's Underdome Riot Add-on for Borderlands has 5 achievements worth 125 gamerscore.
Reach the end of the larger challenge in The Angelic Ruins coliseum
Reach the end of each of the 3 larger challenges with one character
Reach the end of the larger challenge in the Hell-Burbia coliseum
Reach the end of the larger challenge in The Gully coliseum
Complete the Prove Yourself mission