Riddle’s unique magic Off with Your Head (hidden meaning: Off With Their Head) is the ability to temporarily seal away the magic of another mage.
Cater explains, “To a mage, losing the ability to use magic is about as painful as losing your head completely.”
Riddle learned his unique magic at age 10.
Riddle uses his unique magic on many members of Heartslabyul as well as on a ghost, Sebek, Grim and multiple members of Savanaclaw.
During Book 2 Riddle’s unique magic is blocked by Leona’s magic, though Riddle successfully collars him after Leona is distracted by the surprise of Jack revealing his own unique magic.
Ortho explains that he is capable of blocking Riddle’s unique magic with a technomantic energy barrier in Riddle’s dream of Book 7 due to Riddle’s limited understanding of the technology:
Riddle’s signature spell doesn’t work on technomantic humanoids. Well, technically speaking, it’s more that Riddle perceives magic and technomancy as totally separate things. So his signature spell doesn’t treat me as a target for locking down magic…so far, at least.
This seems to insinuate that Riddle actually would be magically capable of cutting off Ortho’s technomantic power, but he was not able to in Book 7 because they were in a world designed by his own imagination. As he did not believe himself capable of doing so, he could not.
Cater explains that Riddle can use his unique magic on multiple people simultaneously, with Trey saying, “His signature spell relies heavily on initiative. If he can land it, he effectively neutralizes his opponent’s magic. That’s why he works so hard on improving his casting speed.”
Referring to the tests performed on Riddle during his internment at STYX, Ortho says, “the bare requirement to beat Riddle in a duel is to cast faster than he can.”
Riddle uses his unique magic repeatedly in Book 7, collaring Leona, Trey and unnamed Heartslabyul students in his dream.
While he later attempts to use the spell on Deuce, it is blocked by Ortho’s technomantic energy barrier.
Riddle also uses his unique magic to collar and defeat a imitation of his mother born from darkness within his own mind in Book 7.
Riddle uses his unique magic during the Nightmare Before Christmas event on Shock and Barrel.
Twisted Wonderland Reference Note 