The dreamthief's daughter - Michael Moorcock

In the 1930s, Count Ulric von Bek has been harried and imprisoned by the Nazis for a black sword which is part of his family's history - and for the Grail, which his cousin Gaynor believes is also in his care. Almost dead, he is rescued from Sachsenhausen concentration camp by two unknown figures - an Englishman called Bastable and an albino girl, Oona. With them, he journeys to a strange, underground world. And there he meets a figure known to him only from dreams, in which they are somehow the same person, yet separate: Elric of Melnibone.
As their stories intertwine, von Bek comes to know something of Elric's past and recent history, and their very beings become one. Sometimes Elric is in control, sometimes Ulric. And the never-ending struggle between Law and Chaos must be fought out in both their universes, with the help of the enigmatic Dreamthief's Daughter. Plaiting differing realities effortlessly, mixing the eternal city of Tanelorn with the rise of Hitler's Germany, Moorcock at last shows us the origin of Stormbringer, Elric's black, soul-stealing runeblade, and Mournblade, its sister sword...