Dragonlance

From Fizban's Treasury of Dragons, page 23.

Description
Weapon (lance or pike), legendary (requires attunement)

A dragonlance is a renowned weapon forged from rare metal with the aid of powerful artifacts associated with Bahamut. Different lances are forged for use by foot soldiers (as pikes) and by riders (as lances), but the magical properties of the weapons are the same.

You gain a +3 bonus to attack and damage rolls made with this magic weapon.

When you hit a Dragon with this weapon, the Dragon takes an extra 3d6 force damage, and any Dragon of your choice that you can see within 30 feet of you can immediately use its reaction to make a melee attack.