Rakdos Riteknife

From Guildmasters' Guide to Ravnica, page 180.

Description
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.