Arcane Memorization

For any spell that you hear and see being cast, you can use your reaction to roll a 1d20 plus your intelligence modifier to remember that spell DC 15. After an encounter you must study that spell for 3 hours to memorize it. You can only remember two spells per encounter. Once you memorize the spell, if you do not have a spell slot to cast that spell, then you gain a spell slot to cast that spell at its original level. If you acquire spell slots of the same level as the one you memorized, you then lose that slot previously gained to cast it.