Mad Moxxi's Underdome Riot is the second piece of DLC for Borderlands. It features three new riot arenas and storage for players' items. The plot of the DLC is that Moxxi, a crazed lover who is setting out to find her 4th husband, has made the arenas for combat. Players fight several of the game's enemies, including bosses, in arenas.
Mad Moxxi wears her heart on her chest.
I'm thinking Moxxi may be related to the Mad Hatter from Alice in Wonderland.