He was mentioned in "Cedric's Apprentice" even though he never appeared in the episode along with the others from Season One.
Greylock is King Magnus's Royal Sorcerer, who is very loyal and talented. He is also known to be Cedric's former schoolmate at Hexley Hall, who would often pull pranks and tricks on him, and even giving him nicknames for the fun of it. He is skilled at powerful parlor tricks, having a sorcery competition with Cedric at the Royal Jubilee while Sofia aided Baileywick. After the competition led to the magic carousel the children rode being levitated out of control, the two sorcerers worked together to bring it back to earth. They later did a similar act in taking the royal schooner, which had lost its steering wheel, back to the harbor.
In Season Four, it is revealed that behind Greylock's prankful exterior he was actually very cruel and desired power. He had absolutely no respect for King Magnus, accepting Grimtrix's offer to join the Order of the Wand without a second thought. He was last seen taking over Rudistan, then was defeated by the other good sorcerers.