The Ring of Sauron

[the phantom]

without the Ring, he could be slain and come back, and slain again and come back again

We don't know that. After his body was destroyed at Numenor's downfall he had lost a portion of his control over his reincarnation abilities.

From Akallabeth-

and though he was robbed now of that shape in which he had wrought so great an evil, so that he could never again appear fair to the eyes of Men

And that's with his ring helping him. It's very possible that he had become fully incarnate and would have died permanently (so to speak) right then and there if not for his ring anchoring him.

The ring allowed Sauron to become more invested in the physical world without fear of death. If he becomes incarnate, no big deal because he has his ring to anchor him if his body is killed. On the other hand if he were to become fully incarnate without an anchor then killing his body would kill him.

The ring was an excellent idea. It turned permanent deaths into temporary vacations and turned military defeats into mere delays before victory.

Seems like a pretty foolproof plan to me.

Yep.

Of course, if someone waves it around at the Crack of Doom they might fall in with it. But what are the chances of that ever happening?

Zero, unless Eru is involved. [img]smilies/wink.gif[/img]