Cybernetic flaming blast (WIP)

Your robotic arm whirs into action, pistons and gears shifting as its wrist shoots upward, revealing a barrel. The joints in its wrist spin to incredibly fast speeds as if generating power, you throw your arm elbow first backwards behind you and the outer mechanical shell of your forearm cocks back. You then violently slam the arm forward in front of you, releasing a devastating blast of fire in a 15 ft cone. Enemies hit by the blast must make a dexterity saving throw. Failing the saving throw will result in the creature taking 2d8 fire damage + your CON modifier, taking half as much damage on a successful save. This ability can be used an equal amount of times as your proficiency bonus per long rest. (The damage increases to 2d10 at 5th level, 2d12 at 11th level, and 3d12 at 17th level).