You gain a +2 bonus to attack and damage rolls made with this magic weapon. 

Embrace the Pain.
When you are under 50% HP, as a bonus action, you may enter a state of Reaping.  This state lasts for 1 minute and it drops if you fall unconscious. While in this state you deal an additional 2d4 necrotic damage and additionally gain resistance to necrotic, slashing, bludgeoning, and piercing damage. If you have resistance to necrotic it is instead turned into Immunity. You can use this feature once before a short or long rest.

Everseeking Dream.
The Blade seeks a dream of an eternal night, when you reduce a creature to 0 hit points with this weapon you have to make a Wisdom saving throw DC 15 or fall victim to its will. If you fail this saving throw you are filled with madness. (Roll on the madness table and see what chaos unfolds.) If you succeed on the saving throw instead you regain hit points equal to your level. This effect is cumulative and lasts till a long rest.

Blade of the Vengeful.
This blade has the finesse & reach property. additionally, it gives you immunity to the frightened condition. While Attuned to Yami No Ha you are able to cast spirit shroud at level 5 on yourself once per long rest.

Sentience. Yami No Ha is a sentient chaotic neutral weapon with an Intelligence of 12, a Wisdom of 19, and a Charisma of 15. It has hearing and darkvision out to a range of 120 feet.

The weapon can speak, read, and understand Common, and can communicate with its wielder telepathically. Its voice is deep and echoing as if hearing from the depths of the shadow fell. While you are attuned to it, Yami No Ha also understands every language you know.

 

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, Immunity: Frightened, Weapon Mastery: Sap (Longsword), Weapon Property: Finesse, Weapon Property: Reach, Damage, Combat, Warding, Versatile, Sap