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 (example: dragons, dragon turtle, half dragon).
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.