I can't remember all the details but I do remember this:
Harry is abused by Vernon and Dudley but Petunia is on Harry's side and takes care of him. Harry is sent out of the house when Vernon has some important people coming over. Petunia gives Harry a backpack with food and water and a sleeping bag in and Harry goes to the woods where he usually goes when he's kicked out.
Harry goes to sleep on a wooden pallet in the woods. He is then found by Fenrir Greyback and taken back to his den and pack.
Dumbledore is evil and has been helping Vernon abuse Harry.
Snape is a Vampire and his mate, Sirius, is "dead" and Remus is trying to force him to drink his blood so he won't die.
Remus' mate Regulus is "dead".
Sirius had fallen through the Veil and Regulus was killed and is also a werewolf.
They are both revived and instantly go to Grimmauld place and find their mates.
Sirius is also an Incubus.
Harry is also the grandson of Voldemort.
Voldemort is good.
This is a story I read on Archive Of Our Own.