Using the sword as an reaction you can roll to see if you can cut a spell (fireball,ice knife ECT) on on a critical (19-20 and also 18 on the dice roll no modifiers) it can absorb the spell and you can cast that spell anytime (no spell slots or items needed does stack)
Proficiency with a Longsword allows you to add your proficiency bonus to the attack roll for any attack you make with it.
This weapon has the following mastery property. To use this property, you must have a feature that lets you use it.
Sap. If you hit a creature with this weapon, that creature has Disadvantage on its next attack roll before the start of your next turn.
Notes: Bonus: Magic, Damage: Slashing, Use a spell while having the sword equiped, Damage, Combat, Versatile, Sap