You gain a +3 bonus to attack and damage rolls made with this magic weapon. In addition, the weapon ignores resistance to slashing damage.
When you attack a creature that has at least one head with this weapon and roll a 20 on the attack roll, you cut off one of the creature's heads. The creature dies if it can't survive without the lost head. A creature is immune to this effect if it is immune to slashing damage, doesn't have or need a head, has legendary actions, or the GM decides that the creature is too big for its head to be cut off with this weapon. Such a creature instead takes an extra 6d8 slashing damage from the hit.
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, Combat, Versatile, Sap
how do you get it
I don't know
If you use the dms guide, then it is one of the magic items you can get. I had a fighter, and I just happened to roll crazy high and end up with this beast