Halo. Vitiate isn't a spirit like (possibly) abeloth He got killed by the hero of tython. Halo rings wipe out all life If he wants to do something like a soul transfer, the body he goes into must be alive. if it dies, he dies. Halos wipe out all life within 25,000 light years each. And their pulse is a shere. So anything within that range dies. Installation 05 has a successful utilization record of 1.2 trillion simulated and one actual firing. With the forerunners, it is makes sense to assume they would be thorough enough to go through the entire process of firing and cooling down before the ring is ready to fire again. That means it can fire 22.8 times a minute As for taking over minds, there is no indication that he can do that to everything/everyone. I'm pretty sure it's not some magical button, but something like a contest of minds. And gravemind is the ultimate contender (or even mendicant and offensive bias) And how does he take out all life? The halo rings can be made fairly quickly. And have been shown to wipe out all life in the galaxy The forerunners can hide in shield worlds, or just sacrifice themselves so that the rings can fire as soon as possible. The AI can continue the battle later