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:
You must have reached level 20 on the first map to unlock the Inlaid Library.
Method:
As soon as you spawn, start heading to the right. 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...