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 Hell-Burbia coliseum
Reach the end of each of the 3 larger challenges with one character
Reach the end of the larger challenge in The Angelic Ruins coliseum
Reach the end of the larger challenge in The Gully coliseum
Complete the Prove Yourself mission