The Potter-Black Heir by Polaris Aries Black
When Harry's hit with the killing curse at the Final Battle instead of going to Kings Cross time rewrites itself. In 1977, Sirius is in a situation where his best friend James will have to pick between himself and Lily. The war wages on outside of Hogwarts and sides are being chosen. Now he not only has to protect his friends and him but the baby that depends on him. Slash.
Harry Potter. Rated M.