Winter’s Dark Bite

Melee Weapon, uncommon

You have a +1 bonus to attack and damage rolls made with this magic weapon.

This greatsword is made of an unknown black metal. In most cases, it works as a +1 greatsword. But when used against a thessalhydra, it works as a +3 greatsword. While in the Upside Down, it works as a +4 greatsword.

Source: Hunt for the Thessalhydra p. 4