The World Undone

Something horrible happens that fateful Halloween James and Lily Potter died. They weren't the only ones to die. Their infant son Harry dies too. But what happens when Sirius and Marlene find out the Potters weren't the only ones to suffer. That the Longbottoms were tortured into insanity and their son dead. What happens next? How do they go on? And will anyone defeat Voldemort?