Blade of Dragondoom

Weapon (sword), very rare (requires attunement)

This marvelous golden blade has a bejeweled golden dragon on the hilt. While holding the weapon, you can understand and speak Draconic. You gain a +1 bonus on attack and damage rolls made with this weapon. In addition, when you hit a dragon with this weapon, the dragon takes an extra 3d6 damage of the weapon’s type. For the purpose of this weapon, “dragon” refers to any creature with the dragon type, including dragon turtles and wyverns.

When you score a critical hit against a dragon, you can proclaim its doom is upon it as a bonus action. This affects the dragon as phantasmal killer (save DC 16), and as long as you maintain concentration on the phantasmal killer you gain resistance against the dragon’s breath weapon.