Can we bring back dead back to life?
If that is possible, what will it use? Prime or purity?
All player characters are Immortals, so the question does not apply.
If you are talking about Creations, e.g. human heroes, that depends. You can Exeunt a character, saying that he died, but this leaves him intact as a creation and he can return. You can also destroy creations, but the rules on that are yet incomplete. A destroyed creation would be unable to return.
Woops sorry I was talking about humans. So being killed and being destroyed are two different things. Is there undead in this world?
Nobody created any undead yet. They would certainly exist in myths. In reality, they would need Prime, so you can use them in your story, but they would not continue to exist outside of it. I should make a rule about magical beings, I guess.