Spellthief Ring

Ring, very rare (requires attunement by a spellcaster)

This ring is made of a strange, iridescent metal with a small crystalline structure at its center that pulses with arcane energy. While wearing this ring, when a creature within 60 feet of you casts a spell of 5th level or lower, you can use your reaction to attempt to steal the spell. The caster must make a DC 16 Intelligence saving throw. On a failed save, the spell fails and has no effect, and you gain one spell slot of the same level as the spell that was cast (or the highest level you can cast, if it was higher). If you have no spell slots remaining, you instead gain temporary hit points equal to 5 times the spell's level. You can use this feature three times, and regain all expended uses when you finish a long rest.