Backbiter

This obsidian dagger has a small opal set in its pommel that seems to glow faintly in darkness.

Backstab. As a bonus action, you can make one melee attack with Backbiter. You can add your Strength or Dexterity modifier to the damage of this attack, even if you have another weapon in your other hand. If this attack has advantage, or if one of your allies is within 5 feet of the target, the attack deals an additional 2d6 damage.

Bloodthirst. At the beginning of your turn, if you haven't hit a creature other than an undead or construct with an attack using Backbiter since your last turn, you take 2d4 piercing damage as Backbiter extends barbs from its hilt and drinks your blood. This property doesn't apply on the first round of combat.

Sentience. Backbiter is a sentient, chaotic neutral weapon with an Intelligence of 12, a Wisdom of 10, and a Charisma of 14. It has hearing and darkvision out to 120 feet. Backbiter can communicate with an attuned creature telepathically in a high-pitched voice. While you are attuned to it, Backbiter understands every language you know.

Personality. Backbiter craves violence, and will urge its wielder forward into battle if it detects enemies. In combat, Backbiter fills its wielder's head with crazed giggling as it revels in bloodshed. Outside of battle, Backbiter acts coy, rarely speaking unless addressed.

Backbiter grows discontent when not regularly bathed in blood. If its wielder completes a long rest without having hit with an attack using Backbiter since their last long rest, Backbiter initiates a conflict with its wielder.

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

Notes: Combat, Sentient, Finesse, Light, Thrown, Nick