The Heucuva, or Huecuva (it's been spelled both ways) is an obscure undead monster hailing from the realms of Dungeons & Dragons. It can be thought of as the clerical equivalent to the Death Knight; a skeletal undead created when a divine character (originally a cleric, but 3e added paladin, druid and monk to the victims list) angers their patron deity through acts of heresy. Most heucuvas are created from fallen priests of good or neutral deities, but an evil priest who angers their deity through incompetence or attempts to seek redemption may also be cursed with this state.
Unlike the Death Knight, or the Clerical Lich, the Heucuva is not a particularly great threat. They retain only dim memories of their former lives - enough to instinctively possess a hatred of living priests and to create lairs in grotesque mockery of true shrines and temples - and minimalistic intelligence. They cannot speak, and can only make simplistic, short-lived plans. They do possess an innate ability to cloak themselves in an illusion of being a living being, but using this to lure a victim in close is about as much as they can think of doing. They are, however, heavily resistant to turning, and their clawed fingers spread a rotting disease... worse, in third edition, unlike in AD&D, heucuvas can still cast divine spells; they're not great tactical users of their spells, but that spontaneous barrage of Inflict Wounds spells can be killer. 3e heucuvas are also heavily resistant to physical damage not inflicted with silver weapons, which does give them a necessary boost.
Unlike the lich and the death knight, the heucuva never really caught on, perhaps because it spent most of its existence as just a tougher skeleton with a diseased claw attack and a "gotcha!" gimmick. Even in 3rd edition, when it gained spellcasting, it just wasn't really really that interesting; clerical liches offered far more versatility and power, so they were better bang for the DM's buck.
Heucuvas first appeared in the Fiend Folio for Advanced Dungeons & Dragons 1st edition. They were updated to 2nd edition in the Monstrous Compendium Volume 2 before being ported to the Monstrous Manual. In 3e, they returned as a template in the 3e Fiend Folio, but also made appearances in issues #86 and #94 of Dungeon Magazine.
It made a surprise reappearance fairly early into 4e; in Dragon Magazine #364, the article "The Ashen Covenant" not only features a huecuva NPC, but also provides the huecuva "template" for converting NPC clerics, paladins and other divine creatures. Here, it was changed so that whilst the "classic" version still exists as a punishment for bungling priests, evil deities have also been known to reward their faithful by turning them into huecuvas who retain their full memories and considerable power; they have a potent Regeneration ability and exude an aura that induces Deathless Fanaticism in their allies.