Unique is an upgrade path in Dark Souls II.
Unique weapons are usually more effective at +0 than Standard weapons of the same class. Such weapons often have higher base damage, superior scaling or both but are improved through Twinkling Titanite.
Some armor also requires Twinkling Titanite.
Improving these items requires Twinkling Titanite, a rare material that is usually found in a very limited quantity. This makes upgrading unique weapons a very time consuming process.
The main benefit of the upgrade path is that it only requires one type of upgrade material, meaning that farming can be done within one area.
Unique weapons can be upgraded to +5 and it takes 15 Twinkling Titanite to fully upgrade. Armor can also be upgraded to +5 and takes 10 Twinkling Titanite per piece, requiring a total of 40 pieces for a four-piece set.