The Pustulous Mass

''"At the end of the hallway, where the throne and the stairways to the rest of the Castle was...A giant, amorphous mass of pustulous flesh which dominated the scene. It had bones and muscles of various creatures, many of which have been warped to the point of unrecognizability, all lumped onto it in many layers as if it was some sort of nightmarish pile of garbage. It had no eyes which could be seen. It continually pulsated color, as if it itself had drained the lands around it [of their color]. Atop it, hanging on a single giant antler, positioned in a way that was almost hilarious, was a rather ornate crown." - Description of the Pustulous Mass''

Summary
The Pustulous Mass was the monster that attacked the Land of the Fae, and which killed the former tyrant king (and presumably incorporated them into itself). It also caused the warping and draining of color of the surrounding area, as if it were a sort of parasite, sucking the life out of everything around it.

It was killed by the combined efforts of the Harlequin Rook, Warren, and 2573, though not before it destroyed the village surrounding the King's Castle and killed the rest of the High Court of the Fae.