15(Shadowhunters)

An Au where Magnus descends from the first warlock from his mother's side(if Asmodeus knew this or not is up to the writer).

Misha Morningstar was the first warlock ever born and Lucifer was his father, he was born much before the shadowhunters came to exist and when Raziel choose Jonathan Shadowhunter to help fight demons, Misha took Jonathan and the rest of the nephilims under his wings(literaly and figuratively as Misha's warlock mark were his wings) so everything the shadowhunters know? They were taught by an warlock.

Abigail, Jonathan's sister, fell for Misha and vice versa so with Jonathan's approval the two married and soon had their own kids(Misha isn't sterile because he is the son of Lucifer).

Unfortunately one of their children got very sick for the overload of angelic blood on them so Misha had to seal the child's blood and power besides him and Abigail deciding that Abigail would raise the rest of their children as shadowhunters while Misha would raise their sealed kid as a mundane.

After the death of his wife and all children, Misha decided to live alone far away from everyone and lost contact with the world.

Magnus' mother was one of the descendants of said mundane child whose own descendants still had a bit of his parents blood but very diluted.

Magnus childhood was pretty much the same but when he killed his stepfather, his magic and blood called for Misha has because of being a child of Asmodeus the blood of Raziel and Lucifer had waken in Magnus' body while the it never happened in the other descendants(and the descendants of the other children of Misha and Abigail have died).

Misha was surprised by this but sensing that Magnus would be in danger if anyone, especially any Greater Demon, found out about how powerful Magnus was(probably capable of rivalling an angels or Lucifer) Misha choose to seal most of Magnus' power and then take him to the nearest Silent Brothers before he returned to his place were fell into a coma-like sleep for how much magic he used to seal most of Magnus magic.

Anything else can go mostly like canon(any other changes on that in Magnus life is up to the writer) until Alec lets Magnus use his power as Alec's angelic power destroys the seals but thankfully Misha put a failsafe that in case the seals are broken, then Magnus would slowly get all his powers back(otherwise Magnus would have killed everyone around him instead of healing Luke) so he can get used to them.

Dealing with Valentine, Asmodeus, Owl, Lilith and others would probably be a bit easier with Magnus being more powerful but it would be interesting if Alec and Magnus took a page from Clace and went on their own adventure to find out where Magnus' 'new' powers came from(if it is before anything else happens after healing Luke, after dealing with Valentine or whatever timeline, it is up to the writer) and met Misha and learnt a part of the erased story of the nephilims that either the clave hide or someone destroyed any traces of it so no one would know the truth.