Okay, I think the problem is the word "immune" because I think you're saying that he (in regards to AM) is just very, very hard to injure, much like some other characters, but saying he's "immune" to all except Valar and Maiar gives the wrong impression. Other characters of similar strength should also be able to injure him, which should be discussed between you and the other player on a case by case basis.

Basically, for the sake of fairness he should be able to be wounded by characters with similar strength to himself, not just Valar and Maiar specifically.

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Re: immune to death - if he's able to be injured he's able to be incapacitated in some way. For other characters who cannot die, we have arranged that they fall into a hiatus-coma while Eru puts them back together. If that works for you, awesome, if not we can discuss another option.