Voldemort wanted a son, having another Sytherin heir at his disposal could prove to be useful. That night Elena Snape bore twins, giving the dark lord her first born son, and hiding her second born daughter far away. 16 years later, Senna Snape is brought to Hogwarts in light of the Triwizard Tournament, and meets a cheerful and clumsy Auror named Nymphadora Tonks. Femslash.