Can be done any time, but easier to locate once you've unlocked the MAP by finding the MILKY WAY MAP relic in the Dairy Plant. Coffins will be marked by a large white question mark (?) on the pause map.
When:
You must have reached level 40 on the second map to unlock the Dairy Plant.
Method:
As soon as you spawn, pause the game and look at the map. start heading towards the question mark - it's a long walk! Keep walking a until you reach the obvious coffin. Destroy the circle of enemies which spawn around the coffin, then walk into it to unlock a new character, and...