8(Shadowhunters)

An Au where everyone is born with a book which you can only be opened when you first meet your soulmate.

Basically once you meet your soulmate your book, which contains the story of your soulmate's life, opens and you can read it and change one single event/decision your soulmate will do in the future that is relates or closely related to you.

Not everyone reads their book once they find out they can open it but some do and do changes.

Magnus had spent more than 400 years without being able to open is book and despite acting and looking like he didn't care about it, deep down he hoped to be able to meet his soulmate someday.

Which happened one night when he was in his club, it was just a look exchange but he felt the moment his book opened and knew that the shadowhunter he had been looking at was his soulmate.

He decided to read the book, Magnus wished to know if it would be worth it to try and pursue his soulmate as shadowhunters rarely treated downworlders as people.

So he read the future of his soulmate, feelings his heart ache and break for what his shadowhunter will have to go through but he also felt warmth at seeing what his soulmate would do to have Magnus by his side.

Magnus didn't change any event as he wished to talk with his soulmate before doing it, he didn't wish to betray his soulmate as many people(unfortunately including himself which he didn't understand as he wasn't someone that would be so petty towards someone he loved even his soulmate) would do so he arranged a meeting with his soulmate.

How the meeting would go, if Alec had read his own book, what events they decided to change(like Magnus could make Alec change the location of the Mortal Cup or unable to be possessed by any demons while Alec could make that Magnus does not do a deal with Asmodeus, etc), other ships, etc is up to the writer(and no Jocelyn and Valentine would not be soulmates...as if they were then things would gone bad very fast).

And yes, I know this idea makes no sense.