9 years after Harry Potter killed Lord Voldermort, there were still a few death eaters lurking around. They pretended to be normal citizens, and were living peacefully, hoping that they would never be found. One minor supporter of Voldermort was Peter McDonald. He was one of the death eaters that hadn't been caught.
Peter was a full blown death eater. He still prayed that somehow his lord would return, just as he did when harry potter was a wee child. But the lord didn't, and as Peter hid, he felt the anger bubble up inside of him. Anger that his master would have been proud of. And he plotted revenge. Revenge on Harry Potter.
Around this time, Harry and Ginny had their last child - Lily. She looked nothing like her brothers, James and Albus Severus. While James was a spitting image of his father, and Albus looked almost exactly like him (he didn't have glasses), Lily looked almost exactly like her mother, but she had bright green eyes - just like her grandmother. And then of course Teddy Lupin lived with them too, as he could not afford a house of his own, and preferred living with the potters above everyone else.
Harry and Ginny loved their children more than anything. They set up extra precautions to protect them - they lived in the completely refurnished house - Grimmauld place, number 12. There was no better way to protect them. No one could even come in unless they had been in the order, or if one of the potters allowed them to come in.
Recently, Ron and Hermoine Weasly were foreclosed, and they, along with their children - Rose and Hugo moved in with the Potters. Ron and Ginny's parents moved in too when they were too old to live alone. George lived in the back of his store, but when he wanted to take part in festivities, he came over for a few days too.
Perhaps it was simply coincidence, but it is believed that Peter McDonald knew what he was doing when he was able to enter number 12, Grimmauld Place (for he had been in the Order of the Phoenix) and he crept up the steps to the fourth floor, and he opened a door. Perhaps he mistook young Lily Potter for her mother, Ginny, but he stopped at that door, and he peered at Lily.
He wanted revenge. He wanted to hurt Harry Potter, but he wanted to do it efficiently and right. So he thought about the spell he himself had perfected, and he raised his wand and pointed it at Lily.
"Tryphean-hiy," he said loudly - he wanted Harry to hear it. He wanted Harry Potter to come running down the stairs for he heard his daughter's screams, and he wanted Harry to find him there. For he no longer desired life and he wished for Harry Potter to kill him.
Lily's screams echoed throughout the house, having the exact effect that Peter had wanted...