Blade of Eldritch Power (5e Equipment)
From D&D Wiki
Weapon (any sword), legendary (requires attunement by a warlock)
This magic blade grants you a +2 bonus to attack and damage rolls with it and a +2 bonus to spell attack rolls when it is used as an arcane focus. Whenever you reduce a creature to 0 hit points with an attack made with this weapon, you regain one warlock spell slot if the creature had a CR of 2 or higher.