Kōsoku (5e Equipment)
From D&D Wiki
Weapon (longsword), very rare (requires attunement)
A black sword, of eastern design. The masterwork of many smiths who worked atop the highest mountain, with this blade, a missed slice is always an opportunity to land another.
Cascade Strike. Whenever you miss an attack with Kōsuko, you roll for damage regardless. Until the end of your turn, you have a +1 bonus to your attack rolls with this weapon for every time you missed with it this turn, and the next time you hit a target with this weapon, you add the damage you would've dealt with the attacks you missed this turn. Once you deal damage with an attack that has missed, you may not add that attack's damage to any further attacks you hit with on that turn.