He is one of the 3 sanin along with tsunade and jiraiya.

His body is destroyed. He found a way to be immortal by taking over someone’s body until their life force is exhausted. So he has to do it every now and then.

He also has the power of curse mark. He can give it to anyone. The curse mark contains a copy of himself that can awaken at the right time and take over the person. He gave it to many.

So basically he’s a human who transcended his humanity and mortality. He is pretty much impossible to kill.