This perhaps is more used in arpgs. For it to work it would HAVE to be optional and available only on non pvp servers.
Another possibility is to just make it an achievement and perhaps grant a title, but no character loss if you die (you are just no longet able to get the title)
Ive seen it done in a mmorpg I play right now. Its quite fun and the whole server gets very exited when someone reaches maxlvl and become "immortal" like other players.
Making it so that it turns back to normal removes any reason to have it in the game.
Its a challange and challanges should come with great risk.