Mannslieb is a white-grey lump of cold rock which affects the tides of the Warhammer World; it is seen as very similar in appearance and size to Earth's own moon, Luna. Mannslieb is a word in Old Reikspiel which means "Beloved of Manann", a name earned due to Mannslieb's having an effect on the oceans' tides (Manann is the Old World god of the sea) similar to that possessed by the Earth's moon. In Warhammer Online: Age of Reckoning, Mannslieb is usually shown as quite a bit larger in the night sky than Morrslieb - this is only appropriate - as Mannslieb is the larger of the two moons.
Mannslieb is known to the Dwarfs as Gormlhune - roughly translated into Reikspiel as High-Moon, or Old-Moon.
In WAR in The Inevitable City, it is possible to jump out to the north from the Arena and come to a portal which transports the player to the Winds of Chaos, an easter-egg region. One of the winds is the surface of Mannslieb.
Morrslieb, also called the Dark Moon, is a giant black and green lump of rock that erratically orbits the Warhammer World. Morrslieb is seen as a symbol of Chaos and unvarnished evil to many of the inhabitants of the Warhammer World. Morrslieb has eclipsed Mannslieb several times during the history of the Warhammer World, an event that always heralds dark times, and the last occurrence of this was just before the terrible Storm of Chaos when the Chaos Champion Archaon's Chaos army attacked the Empire and the Dark Elves attacked the High Elves. However, this event is not part of the canon of WAR, which follows a seprate timeline from the main Warhammer universe. Such portentous times are often (but not always) accompanied by sightings of a twin-tailed comet shooting across the skies, much like the one that signified the birth of Sigmar millennia ago.
In the Age of Reckoning, the parallel Warhammer setting in which WAR takes place, Morrslieb has recently turned blood red as a sign of the impending assault of the Alliance of Destruction and this caused great, fear-driven riots within the Empire.
Morrslieb is said to be made from a giant chunk of Warpstone, though obviously this is unable to be verified. In the Warhammer World, the Skaven, an intelligent race of rat-men, are notoriously fond of Warpstone, large chunkc of which have been known to rain down on the Warhammer World from the skies.
Morrslieb's name translates from Old Reikspiel as "Beloved of Morr", the Old World's god of the dead, and there are a great many myths and legends that explain the Dark Moon's nature. A common legend tells that daemons once preyed upon the people of the world from a gateway in the sky, until Morr inflicted a great death upon them. Afterwards he fashioned the second moon from the wreckage of the daemons, so that mortals would never forget how he had saved them. However the deity Morr, unlike the moon Morrslieb named for him, is not associated at all with Chaos or evil, but rather the ending of life (regardless of the manner in which it is achieved) as well as dreams and portents.
Where Mannslieb is full every 25 days, on a constant and predictable cycle, Morrslieb is not. Indeed, few have been able to accurately predict Morrslieb's cycle, and the moon's appearance varies from night to night, sometimes appearing nearer the world, sometimes further away - it has been speculated by Imperial scholars that it appears differently when viewed from different locations, such that a man perceiving it as full over Altdorf would see it waning were he in Talabheim . In spite of this arcane irregularity, there are only two nights where both Morrslieb and Mannslieb are full during the year: Witching Night (the first day of the year, also known as Hexensnacht in Old Reiksiel, or Hekesdrazh to the Dwarfs ), and the Night of Mysteries (exactly 6 months later, about a month after the Summer Solstice - also known as Geheimnisnacht in Old Reikspiel, or Skraksdrazh to the Dwarfs).
The Dwarfs know Morrslieb as Mhornalhune - translated roughly into Reikspiel as Shadow-Moon.