The third (chosen one)

It was the story about Aries Black Prince. The son of Severus Snape and Regulus Black.

Regulus survived the cave and married Severus, they had a kid, the year after Harry was born, Aries was born the day before the actual boy who lived. But this time Voldemort has chosen the son of the death-eaters he deeply trusted who betrayed.

Defining The dark lord more than three times, Regulus and Severus took their son with them to go on hiding as Lily Potter suggests Peter Pettigrew as their secret keeper. Reluctantly both ex-death eaters agreed.

In one way, it saves both Lily and her husband. But not so much when it comes to her friend.