Sandbox Skull Effect: The Effect of the Sandbox Skull is unknown. There may be no effect whatsoever. Finding the Sandbox Skull will unlock an achievement worth 25GS.

Sandbox Skull Location: The Sandbox Skull can be found on the Mythic map, "Sandbox". This skull is a bit tricky, because it's location is outside of the safety perimeter. Getting to the skull means avoiding death. Create a teleporter node somewhere in the main area of the map. Fly upwards using the monitor and look in the direction of the sun. Turn to the right, past the first pillar, and in between the first and second pillars. The Skull is in this direction at the edge of the map. Create an exit teleporter node, and fly in this direction avoiding fire from the pillars as best as possible, dropping the node directly onto or nearby the skull. You can now teleport directly to the skull giving you enough time to pick it up before being killed by the guardians.