You can read this Fanfic also in:
https://forum.questionablequesting.com/threads/darth-vader-as-severus-snape-hp-and-star-wars.19612/#post-5985000

https://www.fanfiction.net/s/14133981/1/Darth-Vader-as-Severus-Snape

Rewrite of my other Fanfic: "What would you do if you were Severus Snape in his time at Hogwarts"

What if 16 year old Severus Snape had been influenced by more than the dark arts and the loss of Lily.

If Severus had been reached by a being from a galaxy far, far away.

What if Anakin Skywalker or rather, Darth Vader had died when he was left burning by Obi-Wan.

What would happen if when he died only his physical body dies but he remains stranded between the Force and the world of the living because he is the son of the midi-chlorians.

What would happen if he feels that a living being is being attacked by the dark side of the Force and decides to intervene.

And seeing the opportunity to return to the world of the living, he decides to take this opportunity.

And if he were to unite his being with the other person, that would trigger the resurrection of Darth Vader and a new Severus.

Time plays a trick on his life .The future, the past and the present get mixed up.

What does Hermione Granger have to do with all this?

A FF in which you will have romance, action, drama, magic.

An epic search for revenge, Darth Vader versus Darth Sidious.

And two people who do not belong to the magical world of Britain in 1976, but still go ahead and seek to live their lives and improve the lives of others in their way to end the Dark Lord who threatens the lives of all.

There is no system!

This is a Fanfic that only focuses on the canonical world of Harry Potter and Star Wars, all the previous past of Severus and the other characters in the story is canonical, it will only vary and change once Severus starts to change and use his new knowledge.

Severus/Vader, will have a great intellect, talent and creativity for magic and de Force as described in the canon, all this added to his new knowledge, he will be powerful, but it will take quite some time to be as powerful as Voldemort, Dumbledor, Darth Sidious, Yoda or other wizards, Jedis and Sith of that caliber.

There is not much canonical information about the 1970s in HP, so to make Severus have a past as canonical as possible I used all the canonical information about the 1970s and only added information to the blanks left by the canon, so the plot of Severus' past would be as canonical as possible, it may change a little bit due to my intervention in the blanks, but in short, the general trend of Severus' past does not change and the small trend may change.

Disclaimer:
The original content of the Harry Potter characters, books or movies do not belong to me. All belong to their respective creators and owners.

I stated that I do not own the rights to anything other than the FF idea.