Dagger is mundane until attuned. Transforms into a blood red dagger that glows. Looks like a normal dagger to everyone else.

Upon attunement the dagger speaks to the wielder, telepathically. A demon named Vorthas has been trapped in the dagger and he gives the power. 

Wielder gains 5% chance to kill a target they have hit. Once per short Rest.

A critical hit doubles the percentage.

Automatically hits first attack against a creature when hidden

After every failed use the percentage to instant kill goes up by 5%. Resets when used.

 DM/GM ONLY!

(Depends on their relationship they have built up)

Once the wielder has used the dagger's ability they become attached to it permentaly until the dagger succeeds in getting to its ultimate form by killing a certain number of beings (20)

If they try to lose the dagger they must succeed on a DC22 Wisdom save or they fall unconsious and are only awaken by the dagger returning to them or a Greater Restoration Spell. If successfully the are able to cast the dagger away without consequences. 

(Highest number on the dice used)

The dagger was made by the Darrak BloodKith when he made deal with a Demon to kill his enemies and he would serve him. 

Does not work on Immortals or Dragons

After every use of the dagger's ability the more the dagger begins to take over the wielder. 

Ultimate Form - If the user accepts the dagger and allows the demon to be one with them which means they have killed 20 creatues then the dagger can kill Immortals and Dragons. Every time after this that the dagger instant kills someone it has the same chance to kill the wielder.

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

Notes: Damage: Necrotic, Finesse, Light, Thrown, Nick