Nilbog Illustration from Tome of Horrors Complete
So I was browsing through the Tome of Horrors Complete one day, and opened up to the entry about nilbogs. Originally from AD&D's Fiend Folio, nilbogs were a gimmick monster to throw at the party due to a peculiar defense. Possessing a disease wrought by magic warping the space-time continuum, nilbogs actually 'heal' damage instead of being harmed by it. And curative magic damages them instead. Beyond that, they also emit a continual 20 foot radius aura which forces people to roll a Will save or become confused. The damage-reversal is an extraordinary ability, meaning it continues to function even in an anti-magic field or hit with dispel magic.
Now granted, as a monster this is a rather neat encounter in that their weakness is a rather common part of party arsenals (healing magic), but in the hands of a player a nilbog PC would be immune to over 99% of damage-dealing things in the game.
Lo and behold, there are rules for them as PC race, keeping their trademark defensive capabilities. Monster PCs have been a rather attractive option in certain gaming circles, and there was a demand for it among D20 players. It became common in various 'monster manuals' both first and third party to include an "as characters" entry especially for humanoid ones, although in this case this is probably not an option most sane GMs would allow at their table.
Quoted Text for effects:
Damage Reversal (Ex) When struck by any attack that would normally damage the nilbog, it actually gains hit points equal to the damage the attack would have otherwise dealt. A nilbog gains any extra hit points above its normal total as temporary hit points, though it can never have more than twice its normal hit points. (Excess hit points are simply lost.). The nilbog can only be damaged through the use of curative magic and effects (cure light wounds and healing potions, for instance). Curative magic deals damage equal to the amount it would normally heal.
Spatio-Temporal Reversal (Ex) A nilbog constantly emanates an aura of confusion in a 20-foot-radius. A creature in the area must succeed on a DC 11 Will save or be affected as if by a confusion spell (caster level 4th). The save DC is Constitution-based.
Quoted Text for nilbogs as Characters:
Nilbogs as Characters
Nilbogs are defined by class levels—they do not possess racial Hit Dice. Nilbogs have the following racial traits.
–2 Strength, +2 Dexterity, –2 Charisma. Nilbogs are quick but weak and ugly.
Small: Nilbogs are Small creatures and gain a +1 size bonus to their AC, a +1 bonus on attack rolls, a –1 penalty to their CMB and CMD, and a +4 size bonus on Stealth checks.
Fast: Nilbogs are fast for their size, and have a base speed of 30 feet. Darkvision: Nilbogs can see in the dark up to 60 feet.
Skilled: +4 racial bonus on Stealth checks.
Special Attacks: Spatio-temporal reversal (see above)
Special Defenses: Damage reversal (see above)
Languages: Goblins begin play speaking Goblin. Goblins with high Intelligence scores can choose any of these bonus languages: Common, Draconic, Dwarven, Gnoll, Gnome, Halfling, Orc.
Disclaimer: Unbalanced choices are a dime a dozen in third party products (and rather common in Paizo books as well), but what made this one interesting in that it's based off of an official monster from an earlier edition, and being in the Tome of Horrors which is a very well-known and regarded book in the community. I can't see any Game Master in their right mind allowing this as a PC race, but overall the book it came from is a pretty good resource for your games.