Kalran: Tales of the four nations/Arcane and Divine Magic

Curse of Cold Iron
A great curse that befalls anyone who becomes a mage, or born a Sorcerer, and the main reason why they loathe gunpowder weaponry. The curse of Cold Iron manifests whether someone makes a contract with the Fairies, or directly bonds with the Fae realms in the case of Sorcerers, as the Fairies themselves have an aversion to metals. And with this curse an arcane user who touches a metal, whether copper or steel, will hurt them, akin to touching a red hot iron, and it's safe to say then when a user gets hurt by metal, whether knife or bullet, the pain is tenfold, often enough to knock novices unconscious. Thus most mages prefer cloth or leather armor and stone or glass weaponry, and with magic, such materials are more practical to use.

The sole way a mage can bypass this curse is by learning the submagic of metal. And Magus, those who made a contract with the younger Fay, and Warlock, who made a contract with the older Faerie, are able to wield metallic items in small amounts, though the Magus magic is weak to the point that users must use equipment to use their magic while Warlocks must pay a price, an equivalent exchange, to cast their spells.

Trivia

 * The magic system is based off bending from Avatar: The Last Airbender. With casting based off martial arts, like bending.
 * Water: Based off Tai Chi, Aikido, and Taekwondo
 * Earth: Based off Hung Gar, Boxing, Pugilism, Sumo, Wrestling, and Wing Chun with more emphasis on grappling, throwing, and punching
 * Wind: Based off Baguazhang, Savate, and Capoeira with more emphasis on kicking with dance-like moves
 * Fire: Based off Northern Shaolin, Muai Thai, and Krav Maga
 * Some of their sub-elements are based off various magic systems.
 * Blood (water): Bloodbending from Avatar and Blood Magic from Dragon Age. Except not taboo or inherently evil
 * Metal (earth): Metalbending from Avatar and Lore of metal from Warhammer
 * Void (earth): A Fusion of Blackbeard blackhole, Fujitora gravity, and Whitebeard tremor power from One Piece
 * Combustion (fire): Combustionbending from Avatar mixed with Gambit and Yoshikage Kira power from X-men and JoJo