I'm just curious what Deer Knight is based on or is it like an original spell.
It could be based off on the wendigo from Algonquian folklore. According to Wikipedia, the wendigo is a "mythical man-eating monster or evil spirit native to the northern forests of the Atlantic Coast and the Great Lakes Region of the United States and Canada." Many artists depict this creature as deer-like, so it seems the Deer Knight spell could be based off on this.