Unique is an Upgrade Path in Dark Souls.
Unique weapons can be upgraded by all blacksmiths, or with the Weapon Smithbox, by using Twinkling Titanite and the required souls.
Unique weapons can be upgraded to +5 down the Unique Path, increasing the weapon's base damage. Unlike standard weapons, unique weapons do not gain an increase in parameter bonus when upgraded.
Most unique weapons cannot be augmented, the exceptions being the Silver Knight Straight Sword and Stone Greataxe.