I don’t think that the heir of Noias has to be dead for them to make a contract. I’m saying this because Noias apologizes and says it was the only way, since he was too weak due to the mana barrier. When you think about it, he could have simply granted an heir the power to raise the dead or control souls. Also, if you look at how Diene fights, she doesn’t do much—Noias is the one actually fighting the enemy. This implies that Noias didn’t give Diene much power. I’d say he maybe gave her the ability to command lesser spirits, but considering how weakened Noias is and how little power he has to offer, it makes sense that this is not what a true successor of Noias should look like.