One of the two moons of the Warhammer Fantasy universe (the other being Mannslieb), Morrslieb is a giant chunk of Warpstone that orbits the Warhammer world. It came into being when the ancient polar gateways of the old ones collapsed, the resulting storm of warp energies rising up into the night's sky and forming a new evil moon. Due to its Warp attributes, it does not have a predictable cycle but instead randomly appears in the sky at night. Its appearances herald Destruction aligned events, like Chaos invasions or mass gatherings of the Undead.
Lizardmen have also attempted to destroy it in the past, believing that only by ridding the sky of it can they bring about the end of Chaos. So far these attempts have resulted in great harm to the rest of the world, but indeed the forces of Chaos have been weakened in the long run by the diminishing of Morrslieb.
Wood Elves apparently use its power to enchant arrows which are more effective against Forces of Order (i.e. good guys).
In The End Times, with the aid of a scroll given to them in secret by Skreech Verminking, the Grey Seers finally drag Morrslieb closer to the world as part of a plan to regain their prominence. The Slann of Lustria put up a huge fight, resulting in many deaths from magical strain on both sides, but fail to stop the Grey Seers entirely. In a typical display of Skaven arrogance/envy/idiocy, Clan Skyre then tries to one-up the Grey Seers by blowing Morrslieb up with the Morskittar Engine, the biggest warp cannon ever built (it's not clear if they just wanted to blow chunks of the moon so they would get their hands upon tons of meteoric warpstone). Of course, since Morrslieb is so much closer to the world now, this means they nearly end up destroying the fucking world in the process. It takes the heroic sacrifices of Mazdamundi and Lord Kroak to keep the world from dying, and even then, Lustria and the Southlands are both utterly annihilated by a cosmic firestorm.
The surface looks like a skull. A skull.