dukes blade

when attacked you may cast this spell as a reaction. An ghostly air coats a pairing blade in your off hand or materializes in your grip, made of magical force. Until the start of your next turn, you have a +5 bonus to AC, including against the triggering attack.

additionally if an attack including the one that triggered this spell fails to damage you, the attacker takes damage equal to how much they failed by.

eg; if your AC is 12 and you are the target of an attack where the attacker rolled 13 to hit you, if you use this spell your AC is now 17 until your next turn and the attacker takes 4 damage.