Severus Snape
Voldemort, having regained his full strength at the end of "Harry Potter and the Goblet of Fire", summoned his followers to him by touching Peter Pettigrew's Dark Mark. Except for Severus Snape (who was staying at Hogwarts to "maintain his cover") and those dead, imprisoned, or afraid to return, the majority returned to his service as Voldemort began his second attempt to claim all power.