This eastern, fiendish-looking +2 longsword was crafted in the depths of Hell by a master blacksmith. The blade comes with a special scabbard which holds as much power as the deadly sword. It is unknown to mankind or even to fiends if multiple weapon of this kind exist, although it is unlikely. A creature attuned to it can use the following action :

Slashing sheath : An attuned wielder may use the Mark special action (see Dungeon Master's Guide page 271). While the enemy is marked, you count the number of consecutive attacks that hit with Yamato, up to 5 consecutive strikes. While the target is marked, you can use your action to sheath your blade in a ritual-like manner. When you do, you deal 1d8 extra slashing damage to the target. Your consecutive strike count then drops to 0. 

Note : This item is based on the weapon of the same name (Devil May Cry)

Proficiency with a Longsword allows you to add your proficiency bonus to the attack roll for any attack you make with it.

Notes: Bonus: Magic, Teifling, Damage, Combat, Versatile, Sap