The Weeping Lord's Scythe

You imbue your weapon with a very dark, necrotic power. Whenever you hit your next attack with said weapon, the weapon deals an extra 6d6+20 necrotic damage.

If this attack bonus reduces a creature to 0 hit points, your weapon takes the creatures soul and stores it in the weapon, permanently increasing the damage dealt by the weapon by 1d6 necrotic damage. If this weapon breaks, the captured souls are freed, taking away the damage bonus and allowing the creature to be resurrected. Fixing the weapon does not include the souls or damage bonuses.