Malice

+2 Magic weapon.
Whenever you slay a creature with an attack using the dagger, the creature's soul is imprisoned inside the dagger. 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. While carrying the dagger, your dreams are haunted by whispers from the trapped souls.
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.