Bane of the Abyss (5e Equipment)
From D&D Wiki
Weapon (greatsword), legendary
A heavy greatsword forged in the heart of the Abyss, it is said this blade is key to taking down the abyss once and for all. Even while attuned, this magic greatsword deals 2d10 slashing damage on a hit instead of the 2d6 damage for a normal greatsword.
Abyss's Bane (Requires attunement by a character with 17 Strength or higher). While you are attuned to this magic item, every hit with it deals an extra 4d10 radiant damage to fiends, undead, and celestials.