Dragon Crystal Blade (5e Equipment)
From D&D Wiki
Weapon (longsword), very rare (requires attunement from a creature that is not related to dragons in any way.)
This blood red blade was crafted from a rare crystal that causes pain in any dragon type creature. Even the sight of it makes dragons hostile to the bearer.
Anti-Draconic. This weapon does an extra 1d10 necrotic damage to creatures with the dragon creature type.
Hilt Blade. When you take the Attack action and make all your attacks with this blade, you may attack with the hit of this blade as a bonus action dealing 1d4 + your Strength modifier piercing damage and an extra 1d4 necrotic damage to dragon type creatures. You treat this as a normal melee attack.