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).
Every year on Geheimnisnacht (Warhammer Halloween) both moons are full, resulting in very strong Winds Of Magic and the barrier between the material and immaterial worlds being the thinnest, so no matter what faction the most mischief happens during this time from massive vampire and Chaos rituals to Dwarfs and Elves forging magical gear to humans collectively shitting bricks knowing that at least a few Old World towns will be wiped off the map that night. Hexensnacht (Warhammer New Years) isn't as bad, mostly just having the spirits of the dead become restless. Priests of Morr keep the stirring dead in their graves, Witch Hunters continue to work the stirring dead that are trespassing on living property, and intelligent ghosts return. Probably mostly to annoy old men who are too stingy, using Muppets.
In The End Times, with the aid of a scroll given to them in secret by Skreech Verminking, the Grey Seers finally dragged 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 failed to stop the Grey Seers entirely. In a typical display of Skaven arrogance/envy/idiocy, Clan Skyre then tried 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 ended up destroying the fucking world in the process. It took the heroic sacrifices of Mazdamundi and Lord Kroak to keep the world from dying, and even then, Lustria and the Southlands were both utterly annihilated by a cosmic firestorm. The Lizardmen promptly went to space, and everything was destroyed not long after anyway.
The surface looks like a skull in modern depictions, and like a Goblin in older ones. A skull.
Also the Bad Moon, before GW decided to have their cake and eat it too by bringing that back for AoS independent of the 'sleib.