Neither can die while the other survives

Harry is in his 6th year, all events till now are canon compliant, except that, in addition to Sirius's death in Harry Potter and the Order of the Phoenix, Harry has lost Hermione as well. This double loss, along with the emphasis on Voldemort's big return, has weighed Harry down. It is then that he meets Alexa McGonagall, who has secrets she's decided it's time to share.