Morrslieb

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 it's Warp attributes, it does not have a predictable cycle but instead randomly appears in the sky at night. It's appearances herald Destruction aligned events, like Chaos invasions or mass gatherings of the Undead.

Skaven have, 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 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).