Complete act IV.
How to unlock:
Finish Act 4 the Final Act. You cannot miss this playing the main story mode or quests.
Once you leave the main island you CANNOT go back and once you have beaten the Story you start at Chp. 1 with all your stuff but have to replay the story.
Can be gotten in CO-OP (found out today) my friend was playing with me and he got it. The CO-OP must do all the missions with you starting from Chp 1