Leona explains that his unique magic King’s Roar (hidden meaning: same, King’s Roar) enables him to dry things out and turn them into sand.
We receive more detail in the novels where it is described as similar to a natural disaster, drying out insects as they fly through the air while he can kinetically control the sand that he creates and make it move.
Leona says that he was born with his unique magic, which we also receive more information about in the novels as one of the reasons why he has struggled back home (another aspect of his life dictated by his birth that he neither chose for himself nor can do anything to change, no matter how hard he tries):
“Unique magic that is inherited at birth has nothing to do with the person’s will, but humans wrapped up in their own superstitions are ignorant to common sense. Or maybe they think this is a power that I desired, and fought to obtain.”
-Twst the Second Novel
Leona’s unique magic extends beyond drying out objects and the air: he is also capable of turning living creatures such as humans to sand.
Watching Leona use his unique magic in Book 2 Jack says that everything Leona touches turns to sand, but this may have been a misunderstanding of the limitations (or lack thereof) on Leona’s magic.
In Book 6 Leona mentions turning people, phantoms, objects and Grim into sand on accident if he is not careful, and during the Nightmare Before Christmas event he turns magic itself into sand, insinuating that the ability is not limited only to things that he can physically touch.
Similarly, when Ace asks if he cannot turn enemies to sand in a narrow hallway Leona responds, “Not in a place this narrow, unless you guys wanna get sanded too. Do you?”
Leona also uses his unique magic during Halloween, turns ice to sand in Book 6, uses it during his dorm vignette to break up a fight and in Book 3 to destroy Azul’s contracts.
Leona uses his unique magic four times in Book 7: to dismiss a dream projection of Kifaji, to rescue the group from darkness, to save Deuce from being captured in Riddle’s dream and to destroy the magical briars that defend Diasomnia.
Twisted Wonderland Reference Note 