Rune Blade Caster

You have learned how to channel your martial spells through a minor rune inscribed on your weapon.

• You learn how to inscribe a minor version of one rune of your choice available to the Rune Knight archetype in the fighter class at level 3. Whenever you finish a long rest, you can inscribe the rune onto a weapon you're proficient with. Your rune remains on the weapon until you finish a long rest and once you invoke the rune, you can't do so again until you finish a long rest. The minor rune does not grant passive benefits.
  If a rune requires a saving throw, your Rune Magic save DC equals 8 + your proficiency bonus + your Constitution or Intelligence modifier (your choice).
• When a hostile creature's movement provokes an opportunity attack from you, you can use your reaction to cast a spell that involves an attack with the inscribed weapon directed at the creature, rather than making an opportunity attack. The spell must have a casting time of 1 action and must target only that creature.