The Lore of Shadow, also known as "legerdemain" and "phantasmancy", is an arcane magical lore formed from the drives, purposes, and predispositions of the Grey Wind of Magic. It is the lore of illusion, confusion, and concealment. It is commonly referred to as "shadowmancy."
Shadowmancy, then, is the catchall term given to describe the arts of illusionism, which is the branch of magic dealing with the deception of the mortal senses. The Grey Order's shadowmancers seem to bring nightmares alive and the sweetest dreams into reality. They are masters of appearances and sculpting lies. - Warhammer wiki