Robert Crasis is a sorcerer, who appears in The End Of The World. He is told to be the only Adept in Ireland with a particular ability to create an illusion which will make people think that you're somebody else. An example of his work was when he made Mercy Charient look like a middle-aged woman called Francine.
He helped the Sanctuary in The War. Skulduggery mentions that they managed to fool the enemy that there was a battalion waiting for them when there were only a few mages present, courtesy of Crasis' magic.
He is forced to help Foe's Gang and use his magic which he has not used in years, as he wanted to age with his family. He mentions that he had been hoping to never see Skulduggery again, hinting that he has had enough of magic.
His discipline is illusions. He has the ability to create an illusion which will make people think that you're somebody else.