Please hear me out before criticizing the idea.
I got the inspiration for this idea when I discovered that in D&D, Archliches do exist and are of non-evil alignment:
http://forgottenrealms.wikia.com/wiki/Archlich
http://www.wizards.com/DnD/Article.a...d/4ex/20090410
Could it not be an endgame achievement for an arcane spellcaster after level 20?
The benefits I would envision:
Permanent deathblock.
Immunity to disease, poison and fatigue effects.
Permanent natural resistance to elemental damage equal to resist energy at the 11th level.
Player is healed by negative energy spells.
Player can heal themselves by using their own negative energy spells like death aura and negative energy burst.
Upon "death" by melee, ranged or spell damage, player is not bound to a soul stone and can thus travel anywhere within a dungeon to resurrect at a shrine.
The drawbacks:
Player must reach level 20 in their casting class with no multiclassing.
Player is damaged by positive energy spells and potions such as "cure light wounds", etc.
Resting at a shrines will not restore healthpoints, only spellpoints.
Player can never again reincarnate their character.
Please tell me if this would be insanely overpowered or underwhelming and your reasons why.