Deuce’s unique magic Bet the Limit (hidden meaning: Retaliation) is the ability to store up magic that has been used against him and send it back, hitting a foe with the same damage that they inflict on him.
Due to the English-language localization there is a common misconception that Deuce’s unique magic doubles the power of the damage that he takes when he sends it back, but this is not the case.
Deuce explains, “That spell stores up all the damage I take from someone, then hits back with it all at once,” and he did not actually know what it was when it first manifested. He says that, afterwards, he tried to do it again but it wouldn’t work until the battle against Vil in Book 5.
Despite calling out the spell’s incantation and name when he invokes it, he has no memory of doing so.
We see Deuce’s unique magic manifest for the first time on-screen when he is being beaten up by bullies with magic.
The bullies begin to attack Epel and magic begins welling up inside him, though he seems to have no idea what is happening to him before, during or after the process.
Deuce’s unique magic makes another appearance in the game during Glorious Masquerade, when Azul borrows it and then takes a direct hit from Rollo, explaining, “I saved a special present just for you and loaded it with all the nonsense you put me through.”
※ Note: The incantation of Deuce’s unique magic is written in two different ways on EN, but is always the same in the original game. More here ▶︎
Deuce also uses his unique magic in Book 7 to awaken Riddle.
Twisted Wonderland Reference Note 