Sign of the Unicorn by Roger Zelazny

Third in the Amber series, sequel to: THE GUNS OF AVALON. A traitor has revealed the royal family's greatest secret: the ability to manipulate Shadow and travel through all possible worlds, enabling a plot toward Amber's destruction. Corwin, Prince of the Blood, must risk everything in a deadly assault on evil's stronghold. Now, beset by demonic powers beyond imagination and betrayed by a bizarre conspiracy among his own kin, Corwin walks a perilous path toward ultimate knowledge and terrifying discovery.