Source: Guildmaster's Guide to Ravnica

Weapon (dagger), legendary (requires attunement)

You gain a +1 bonus to attack and damage rolls made with this magic weapon. Its blade is cruelly serrated, and its hilt resembles a demonic head and wings. Whenever you slay a creature with an attack using the dagger, the creature's soul is imprisoned inside the dagger, and that creature can be restored to life only by a Wish spell. The dagger can hold a maximum of five souls.

For each soul imprisoned in the dagger, your attacks with it deal an extra 1d4 necrotic damage on a hit. While the dagger is within 5 feet of you, your dreams are haunted by whispers from the trapped souls.

The dagger has the following additional properties.

Siphon Vitality. As a bonus action, you can release any number of stored souls from the dagger to regain 1d10 hit points per soul released.

Annihilation. If the dagger holds five souls, you can use this property: As a reaction immediately after you hit a creature with the dagger and deal damage to that target, you can release all five souls. If the target now has fewer than 75 hit points, it must succeed on a DC 15 Constitution saving throw or die. If the target dies, you can't use this property again until you finish a long rest.