3x21 - Poltergeist: The Legacy

The Legacy is under siege and their assailants are unknown. Alex has gone for help, Nick has been taken down and away by their attackers leaving only Derek to decipher who's behind their attack. As he remembers past foes who may have come back for revenge Derek searches for Nick and guards his own back from threat. Further into his search the clues to who is trying to bring them down becomes obviously clear when Alex returns not quite herself and Derek comes face to face with the foe he should have known all along.