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Mercury elementals are a new type of elemental comprised of pure mercury, they were first introduced in World of Warcraft: Cataclysm. It's unknown if this is a naturally occurring elemental or something that requires a ritual to become. In Deepholm a Twilight's Hammer member performs a ritual by feeding on Mercurial Oozes in order to "ascend" to another form. This process would transfigure his body into one of pure quicksilver.