Nilbog

This article contains something which makes absolutely no logical sense, such as Nazi Zombie Mercenaries, Fucking Space Orangutans, anything written by a certain Irish leper or Robin Crud-ace, or Wizards of the Coast hiring the fucking Pinkertons over a children’s card game. If you proceed, consider yourself warned.

"They're eating her... and then they're going to eat me... OH MY goooooooooooooood!"

– Arnold

Nilbogs are a species of Goblinoid from Dungeons & Dragons who first appeared in the pages of White Dwarf #6 for Advanced Dungeons & Dragons 1st edition, from which they made it to the Fiend Folio. Nilbogs are strange "backwards goblins", said to be infected by some mysterious temporal ailment, or perhaps to hail from some otherwise undiscovered plane; as a result, they take damage from healing spells and are healed by attacks.

Oft-forgotten is that the "spatio-temporal abnormality" that created nilbogs (portrayed as a disorder called, appropriately enough, "nilbogism") was, in fact, much, much more annoying than the oft-remembered meme of "cure light wounds" as an attack and vice versa: in the presence of nilbogs, says the original source material, "the adventurers will have no control over their own actions and will generally pursue courses of action contrary to their normal intent; for example they may feel an overwhelming compulsion to load all their treasure into an empty treasure chest in the nilbog lair and leave empty-handed. There are no saving throws against these effects, nor is there any known defence (though a powerful spell such as a wish, will, if used properly, have a good chance of rendering local immunity against the effects.)"

Nilbogs were, frankly, kind of annoying and fell into obscurity from there. Necromancer Games brought them back for 3e in the splatbook "The Tome of Horrors".

It wouldn't be until 5th edition, in the splatbook "Volo's Guide to Monsters", that nilbogs would return. Here, they are presented as avatars for the spirit of the last surviving member of the lost goblin pantheon, a shard of a trickster deity that possesses goblin hosts and uses them as tools to punish others for repressing its people... especially the hobgoblins. This version changes how it's ability to be healed by attacks works. It is no longer harmed by healing spells, but it can turn damage from attacks into healing only once per round because this ability is a reaction instead of a constant effect, but it still is tricky to damage because it can also charm anybody who tries to attack it.

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  The original, from White Dwarf's Fiend Factory.