Lichslayer

Weapon (any), legendary (requires attunement)
  The faces of trapped souls can be seen floating beneath the polished surface of this deep, inky glass. The secrets of fashioning soulveil, the faceted material from which these weapons are made, predate even the spellbreakers. Before wielding such a weapon against a magic user, the target must have first been tried by a spellbreaker council and found guilty of the most dire crimes. Death by this weapon is an ultimatum, a final curtain.
  You have a +3 bonus to attack and damage rolls made with this magic weapon.
  Soultrap. When you kill a creature that has the Spellcasting or Innate Spellcasting trait with this weapon, the creature’s soul is drawn into this weapon and trapped. The weapon gains a number of charges equal to the half the slain creature’s total number of hit die (20 maximum). As these charges are consumed, the creature’s soul is shredded.
  Empower. As a bonus action you can expend up to 5 charges when you hit a creature with the weapon. For each charge expended, you deal 1d8 additional force damage. When you use the ‘Empower’ feature, roll a d20. If the result is less than or equal to the number of charges expended, roll on the sorcerer’s Wild Magic Surge table.
  Rare variant: Reduce the bonus to attack and damage rolls to + 1, the maximum number of charges to 10, and decrease ‘Empower’s’ bonus damage to 1d6 per charge.