It is well known that when a swan dies, it will sing a song of incomparable beauty.
Some base villains have tried to kill swans just to hear their song. Many of them have fallen afoul of a swan-maiden.
Swan-maidens, when not taking the form of a swan, are grim shadows of blood and fire. The name 'maiden' refers to their asexual method of reproduction, not to beauty. If they do not dismember their foes with dark magic, they have been known to posess them by burrowing into their spine, forming a 'marriage' wherein the victim's corpse becomes a puppet of the swan-maiden.
After some months the swan-maiden causes the corpse of the victim (who is called the husband, although they need not be male) to construct a cloak of swan feathers and its own skin. The husband then performs a ceremony which both summons a new swan-maiden, and destroys the husband's body.
It remains a great mystery why such foul creatures would dedicate their entire existence to protecting swans.