Roger Zelazny's Shadows of Amber

All Hail King Oberon! At last, Dworkin has created the magic Pattern, and Amber has been established with its own myriad of shadow worlds. Now, King Oberon works on repairing Castle Amber after winning his epic battle with the forces of Chaos. And his world is at peace ... until he is attacked by a nightmare creature that refuses to die, while everything in its path withers and turns to dust. Oberon leads the monster away from Amber, on through an almost infinite number of Shadows, and still the creature presses ever closer. At last, Oberon travels to the world of the Pattern itself -- his source of power -- to make a stand against it. His fight, from the centre of the Pattern, rips the world asunder -- even damaging the Pattern itself. But Oberon manages to defeat the deadly monstrosity... and the Pattern repairs itself. Still, much time has passed, and much damage has been done. Among other things, when Oberon returns to Amber, his father tells him that the Pattern has been magically transported to the bowels of the castle. Further examination reveals that the original Pattern remains where it was drawn, and Castle Amber now possesses its identical double. But more damage than that may have been done. In a single evening, Oberon discovers that Amber hosts another race of creatures -- a whole civilisation living beneath the ocean in a mirror duplicate of Amber. He learns that it, too, has had a copy of the Pattern appear in its depths, that the Queen of the undersea kingdom knows all about him and desires him to get rid of the Pattern from her realm; and that she has plans for him... and the powers to perhaps make her wishes real.