Borderlands 2: Creature Slaughter Dome DLC has been part of the pre-order bonus offered by GameStop a few months ago.
However, the add-on has just been launched on Steam and is now available as separate purchase to all players that did not pre-order Borderlands 2. The game is available for only €4 here
Creature Slaughter Dome is a special arena in the Wildlife Exploitation Reserve where players must face various types of monsters and receive rewards.
According to GearBox
, players will also “get to wield a special weapon called Moxxi’s Rocket Launcher, a tribute from the Mad Moxxi's Underdome Riot DLC originally released for the first Borderlands.”
Although players will probably get to the Wildlife Exploitation Reserve at level 17, it is recommended to enter the arena at level 20, for better results.