The agent of the arena
This quest will be offered to you by a 'Shady looking Cornac man' in the north section of Derth. Choosing to fight in the arena will pit you against three foes (a Slinger, a Gladiator, and an Arcane Blade). Defeating all of them will unlock the Arena Campaign, as well as granting you two bonus Generic Points.
The shady Cornac leaves Derth when you reach level 14, when Storming the city begins. If you are going to do the arena quest, you must do it before then.