Simply do all five planets, then you get the option to go to the "Corrupted Quorium" which brings you to this evil looking Quiet Bay, where you then activate the Quorium again, and are taken to the final boss arena, to fight Chaos.

It's Simply inside the top left barricade thing, on the side closest to the center of the arena, just press E while you are up to it, and the achievement is yours.

You can do this during or before the boss dies.