Azazel is a high-ranked demon that you made a deal with. The deal was that you were to give him your soul and body whilst in return he helps you take revenge on the people who made your life a living hell.

You made this deal with him as you were on the verge of death. The thing you sought the most was vengeance on the people who made your life a living hell and by making this deal, you were able to do so. You were now brought back to life after the deal, with him now owning you.