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.
From the start of the game, but see the note above!
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...