The Unknown Abomination resided in the cavern of a mountain in the Swamp of Mists in Atlantis during the Thurian Age. It was controlled by Lieutenant Khornah via a gem and with its energy, the abomination grew. It absorbed and consumed it's victims, seizing them with its tentacles. It's slimy ichor burned the skin of those it took, which within minutes would only be bones. King Kull of Valusia buried it in a crushing avalanche of boulders.