Hydaelyn

You gain the ability to magically imitate the knowledge of how to cast a spell from another spellcaster. Immediately after a creature casts a spell that targets you or includes you in its area of effect, you can take a Reaction to force the creature to make a constitution saving throw. On a failed save, you mimic the knowledge of the spell if it is at least level 1 and of a level you can cast. For the next hour you have that spell prepared and may cast it 1 time without using a spell slot, any casting of it after the initial use will require a spell slot of that level to be used. If the spell you are copying is of a spell level higher than 4th you must spend a spell slot of equal value to the spell you are copying.