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.
When:
From the start of the game, but see the note above!
Method:
As soon as you spawn, start heading to the top-right. (I recommend following the pause screen map, it's much easier. Find the relic in the DAIRY PLANT to unlock the map.) Keep walking a long, long way 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...