Double Dragon

Weapon (dagger), very rare, requires attunement

Double Dragon is a pair of daggers with a silver hilt and black blade. When used together for two-weapon fighting, the first weapon attack roll is always with Advantage. If used against a dragon, both weapon attack rolls are with Advantage

The daggers are especially attuned with its owner — at 5th level and every 5 levels, they gain a +1 enhancement bonus to attack rolls and damage (for a maximum of +4 at 20th level).