Beautiful, but deadly the Sirens are immortal, but are not Daedra. They are Daemons who travel from world to world seducing and sucking the lifeforce from those they prey upon. They are primarily shapeshifters, and are very mischievious. They lull their victims into a stupor with their spellbindingly beautiful songs.
Being able to travel to other worlds has given them the opportunity to learn many new forms and spells of magicka. When a siren is killed it returns to the world it was birthed on. This continues on until a siren has died eight times, on its ninth death it is dead for good. When they are in a rage they look upon their enemies with Hell's Gaze which causes them to flee in terror.