Gra'thok the Devourer

aka Lord of the Kelpies, Harbinger of the Dark Knight, First and Last of his Name, Terror of the Forgetful Swamp, Bearer of the Undying

A legendary beast, Gra'thok is a name only whispered in The Feywild for fear of summoning its bearer. Considered to be a herald of disaster, the kelpie is a king of his kind and other swamp killers swear fealty to him out of fear. It is said that he is the largest of kelpies, with eyes of blue flame like a will o'wisp, and wears battered black armor trailing tattered remains of some dark standard.
Many argue that he is simply a legend, but any Feywild native will insist otherwise. Not only is he real, but he is timeless and impossibly old– the Harbinger of the Dark Knight appears in several texts dating back to the age of Flood and likely existed before that as well. It is said that while he is terrible and fierce, as are the riders he chooses, he will only deign to carry the truly honorable. A curious legend, given that every so-called Dark Knight across history has been controversial at best, and reviled at worst. Some apocalypse cults even revere him as a prophetic symbol of the destruction of the world. Either way, the Devourer has mercifully not been seen in the Material Realm since the days of the Asmodean warlord Dread in the age of Crystal.
It is, however, true that the Forest of the Geas holds an unsteady boundary with the Feywild, and within its labyrinthine trees, swamps, and bogs the mythical Forgetful Swamp has been known to appear… and not many have returned to tell the tale.