Unique

Unique items are special items that can have different and/or more item properties than normal items of the same type.

Any equippable item dropped by an NPC, including even level 1 ammo, has a very small chance of being unique, and if any enemy has a Unique item, then said enemy will always drop that Unique item upon death if you don't remove it from the enemy first.

Unique items have been known to have the following changes compared to the normal version of the item:

-Any existing item property being replaced with a similar one, excluding the attack/enhancement bonus/AC bonus being changed for something else and physical immunities, resistances, and damages becoming non-physical and vice-versa, but certainly including the potential for all charges of all(not some) spells to be replaced with a spell per day... whenever a Unique item spawns, the only difference will probably be an existing item property being replaced with a similar one, such as +7 concentration becoming +7 persuade, +1d8 acid damage becoming +1d8 cold damage, or 25% sonic immunity becoming 25% fire immunity.
-An extra ranged damage type, where it didn't exist before; if the weapon spawns with a light, however, such as Light Hurler, then it can't spawn with an extra damage type.
-An extra +5% to an immunity, physical or not, where it didn't exist before. Divine might possibly be excluded.
-A bonus feat, where it didn't exist before.
-An enhancement bonus that is 1 higher than the normal one vs. a racial type or alignment; attack bonus will be chosen instead if the weapon had an attack bonus instead of an enhancement bonus.
-An AC bonus that is 1 higher than the normal one vs. a racial type or alignment.
-A damage bonus that did not exist before.
-A damage bonus versus a racial type or alignment that did not exist before.
-Regeneration or Vampiric Regeneration where it didn't exist before.
-A skill bonus that didn't exist before; up to +8 has been observed, specifically on a level 30 item.
-A bonus spell slot that didn't exist before.
-A bonus spell, charges or per day, that didn't exist before.
-An increase to an already-existing saving throw, at least versus a specific descriptor such as death or fear.
-An added saving throw vs. a specific descriptor that didn't exist before.
-A ranged weapon will always have an extra +2 mighty, if applicable, in addition to some other unique property, whether or not that would be an existing property being replaced with a similar one.

If you don't like the properties of a Unique item, you can permanently transform it into the normal version of the item by putting it on the Anvil in Avendell, but, by doing this, you can never get back the uniqueness of the item.

Additionally, equippable Valuables such as golden crowns and black mantles are perhaps exempt from being able to be unique.