Every person is born a twin. One is evil and one is good. Your twin died at birth; the government is t sure which one you are. They make their best guess and send you to one of the territories when you turn sixteen. They choose wrong;
Prompt -- Every human has a personal invisible guardian, summoned by magic words long forgotten. You've just said the words by accident. - Lohikaar taught her children the spell and each summon theirs in their most dire need - stars?
But make him have sudden superpowers that corrupt him and eventually one bad thing turns to the next( ie cheating to stealing to something worse.) and they become the villan while the friend sees this and tried to stop it.
Plot twist: his attempts succeed, but the girl turns 35 and refuses to marry him for whatever reason (figures it out, just goes back on the promise because she doesn't love him that way/ decides she doesn't need a man to make her life worthwhile, etc)