I was running off the Underworld movies-- if someone possesses the Corvinus immortal strain, i.e. is directly related to Alexander Corvinus, they're a separate species from Vampires and Lycans. Eve, from the fourth movie, is the canonical character. And as for how she gets out... I was figuring on her either changing while conscious because of some kind of emotional stressor, or that an escape attempt by someone else led them her way and let her out. And dammit I just realized how close that makes them as characters. Hmm.