The black swan - Mercedes Lackey

Baron Eric von Rothbert is a powerful sorcerer who devotes his life to capturing women who inspire his wrath and transforming them into swans. Over the flock he sets his daughter Odile, gifted in magic, but still spurned by her father.

Odette, noblest of the Baron?s enchanted flock, wrests from their captor a promise which could lead to fredom for all the swan-maidens, but the Baron is determined to use all his magical cunning to avoid honouring that promise.

Prince Siegfried comes upon the swan-maidens as day dissolves into dusk -- and the game takes on a new twist, for Siegfried is a selfish hedonist and dissolute womaniser . . .