It is the year after Harry left school, just after the final battle with Voldemort in which all of the Death Eaters died as well as Voldemort himself. But the Order had heavy casualties as well. Ron Weasley, Hermione Granger, Ginny Weasley, Minerva McGonagall, Nymphadora Tonks, Kingsley Shacklebolt, Molly Weasley, Arthur Weasley, Cho Chang, Neville Longbottom, Dean Thomas, Seamus Finnigan, Luna Lovegood, George Weasley, Fred Weasley, Bill Weasley, Remus Lupin, Charlie Weasley and Hagrid, all dead. At the end Harry was just sitting there with his head in his hands. That is when the Dementors descended out of the darkness.
Once Harry had seen them he cast a Patronus. Realising that this one Patronus wasn't enough he cast another and he got the shock of his life – instead of a stag a big, shaggy dog erupted from the end of his wand. He cast another, this time a werewolf. He cast again, and got another shock a Patronus of his dad came from his wand. He called, "Expecto patronum" – his mom; Sirius Black; Dumbledore; Remus; Hermione; Ron; Ginny; Minerva; Tonks; Kingsley; Molly; Arthur; Cho; Neville; Luna; Hagrid; Bill; Charlie; Fred; George. All these patronuses formed a barrier facing inwards while keeping all the Dementors out. Harry looked round at all the faces around him and he felt more despair than ever before. He just wanted to be with them. He looked at his wand for a minute then put it to his temple. After muttering two words he fell backwards and all the Patronuses faded away. He just became another body on the battlefield. The two words, Avada Kedavra.