Sword of the Tourmaline

Weapon (any sword), very rare (requires attunement)

This weapon appears to be a +3 longsword and identifies as such.

Curse. You gain a –3 penalty to attack and damage rolls made with this weapon. Once you have become attuned to the weapon, it cannot be removed from your hand. Attacks made with weapons in the other hand are made with disadvantage. The attunement can be removed by a remove curse spell or similar magic.