Discs of Tzeentch

The Discs of Tzeentch float in the clouds of swirling energy that makes up the Realm of Chaos, drifting through the Aethyr, feasting on lower Daemons and the souls of the damned. They are formless things of shadow on this mad plane, vicious and uncaring. They are commanded by their dark master to seek out and destroy the essences of mortals who are pulled into the plane and retrieve their souls for Tzeentch to change.

Sometimes, Tzeentch despatches these creatures into the mortal world. Once out of the Realm of Chaos, their magical bodies assume a strange physique. Most become round and flat, capable of flying through the air much as they did in their native plane. The magic that transforms them always gives them some other alteration, some mutation that sets them apart from others of their kind.

The most honoured of Tzeentch’s Sorcerers, called Thrall Wizards, are granted a Disc for their constant service. These Discs bear their riders into battle, giving them a unique perspective on the movement of their armies, and enabling the Sorcerer to be better able to strike down important targets. It’s said that since Discs are in essence sent by their God into the mortal world, they can return to the Realm of Chaos at will—sometimes still carrying their riders on their backs.

Source
Warhammer Fantasy RPG 2nd ED -- Tome of Corruption (pg. 231)