One of the two moons of the Warhammer Fantasy universe (the other being Mannslieb), Morrslieb, Old Reikspiel for "Beloved of Morr", is a Warpstone satellite orbiting the Warhammer world. It came into being when the ancient polar gateways of the Old Ones collapsed, the resulting storm of warp energy launched a massive Warpstone chunk into the night sky forming a new evil moon. Due to its Warp attributes, Morrslieb does not have a predictable cycle, it randomly appears in the night sky further or closer than before. Its appearances herald misfortune, especially when closer, including Chaos invasions, mass gatherings of Undead, and individuals to entire villages vanishing overnight. Stranger yet, it is hinted that Morrslieb could have life, such as Moonclaw having been birthed from a Morrslieb meteorite and Be'lakor claiming to have ruled it as a god.
Due to its radioactive green light, it can corrupt the flesh, inspire murder and madness, thin the veil of reality, and warp the unborn. The latter makes it possible to assume it is the primary cause of Beastmen birth, for many human fetuses were corrupted by its light when their mother were ignorantly exposing their womb to it. Midwives staying out of Morrslieb's glow can prevent mutations, but when Morrslieb shines fullest, it can touch the womb in some way, so midwives will attempt to delay gestation until morning.
The surface looks like a skull in modern depictions, and like a Goblin in older ones. A skull.
Skaven, who call it Morskrit, consider Morrslieb important for the Vermintide holiday and that every one of its Warpstone meteorites a glowing gifts from the Horned Rat. They had also at various times in the past used their magic to bring down chunks of it to harvest: this resulted in the destruction and events surrounding Mordheim.
Lizardmen, who call it the False Moon, have attempted to destroy it in the past since they believe deleting it from the sky of will bring about the end of Chaos. So far, the Slanns attempts have resulted in great harm to the rest of the world, but the forces of Chaos have been weakened in the long run by the diminishing of Morrslieb.
Ancient Nehekharans, who called it the Green Moon, associated it with Sakhmet, who out of envy of the Nehekharans due to their favor from Ptra, would send spirits and monsters against Nehekhara whenever she gained pre-eminence over Neru. In Nagash's novel, many Mortuary cultists has tried to predict its sighting in order to harness its magical power. In life, Nagash had come the closest to predicting its movements, using the accumulated knowledge of the Mortuary cult and his own complex formula added into it. An entire volume of Nagash’s tomes was dedicated to his observations.
The Vampire Counts follow Morrslieb's course for its fullness hastens Dark Magic accumulation necessary for Necromancy and makes raising the dead much easier. Only when Morrslieb is full can certain necromantic rites become possible, like the creation of an Abyssal Terror.
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 sheltering their livestock before 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 Skryre 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 off 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 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.
It's unknown how the physical destruction of Morrslieb affected Moonclaw and his steed Umbralok.
Well, turns out Morrslieb didn't end with its physichal destruction. Age of Sigmar 3rd Edition confirms that the fucker's spirit survived the End Times, and now orbits Shyish with a new name: Lunaghast, The Moon of Dark Secrets. Described as a skull-faced planetoid of dark matter, so its appearance's the same, but even more ominous and creepy.
Often seen in the Eightpoints, but having its influence felt across all the Mortal Realms, whomever it shines down upon is “blessed” with unfathomable knowledge, of the mind-breaking Lovecraftian kind. It has a rival in the form of the Bad Moon, another supernatural celestial object worshiped by crazed belligerents. The two have actually been the catalyst for entire wars for their respective followers. Also apparently the Bad Moon actually bit a chunk out of Lunaghast upon seeing it for the first time.
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.