Morslieb is a chunk of warpstone... which has sentience! as it doesnt obey any normal movements of an orbiting moon, it actually moves however it likes and can even appear during the day to menace people.
Empire folk dont leave their homes when Morrslieb is out and even go as far as boarding up there windows and doors, for when Morrslieb is out and full on displayed, mutants, monsters and other terrible creatures come out to play!
If you guys like to know more about this, i would suggest buying the WFRP books, has so much background in them!
Don't, after rolling that final 1 too many, flip out, climb up on the table and jump up and down on your opponent's models screaming 'APOCALYPTIC BARRAGE, MOTHERFUCKER!!!'.