Nightingale Armor

The Nightingale Armor is set of Light Armor given to you and Brynjolf by Karliah before performing the initiation ritual of the Nightingales during the Trinity Restored quest for the Thieves Guild. All of the armor pieces are obtained from a box with the Nightingales symbol emblazoned on it. The Nightingale armor cannot be replaced if lost.

The armor container in the developers' room (accessible on PC only, the command is coc qasmoke) contains three sets of Nightingale Armor with varying armor ratings and enchantment strengths. The BaseIDs given are of the middle set.

Magic effects
Wearing all four pieces of Nightingale armor grants the effect "Nightingale Armor Full Set" which increases armor rating by 25 points. This bonus applies after the perks Custom Fit and Matching Set.

Improvement
Nightingale armor can be improved using void salts. There are six levels of improvement Fine, Superior, Exquisite, Flawless, Epic and Legendary. When combined with a high light armor skill and perks, this can easily exceed 567, (or 80% damage reduction), with a high smithing skill, fortify smithing potions and enchanted smithing apparel.

Bugs

 * If the armor is worn on a Khajiit/Argonian the tail will slip through the cape.
 * Sometimes, Nightingale armor can't be upgraded even when you have sufficient smithing skill, smithing perks and void salts. (This could possibly be a result of having the fine-cut void salts in your inventory, turning them in to captain of the ship in Dawnstar should fix the bug)
 * Sometimes the coloring of the Hood will go and it will become white (PS3 confirmed, 360 confirmed but only when equipping in shadows, others need confirmation). Reinstalling the game seems to fix this. Another way to fix this is to drop it on the ground during the day time and wait until night, pick it back up and equip it.
 * While wearing the gloves, human fingers skin tone will always look the same no matter your character's skin tone.
 * There is a bug when you try to retrieve the armor this way, when you click the stone nothing happens.
 * Argonian and Khajiit faces are shortened when wearing the hood and look like normal human head models.
 * If you obtain the armor from the container and then kill the NPCs, you'll get to keep the armor without continuing the questline. However this will stop you from getting the skeleton key and/or finishing the thieves guild quests.
 * If you obtain all smithing perks except for the last optional one, the Nightingale armor will only upgrade to flawless. (Getting the last perk seems to fix this)
 * 'Giving' the armour to one of the armor manniquins in Proudspire Manor, then removing it, will sometimes result in a duplication of some of the pieces of the set (confirmed on multiple PC playthroughs; awaiting confirmation on consoles)
 * It's possible to use manniquins in Proudspire Manor to duplicate all pieces of the set on Xbox360. This only seems to work with the manniquin on the right. (the manniquin on the left seems to not register the hood)
 * When given to a follower who has heavy starting armor, you may not be able to get it back because it's "Your followers starting armor." The only way to get it back from your follower is to kill them and loot it back.
 * To upgrade the set past flawless, you need enchanted smithing items/smithing potions and 100 smithing. This may depend on what level you are when you obtain the set. (Level 20+ requires 100 smithing+enchanted items)
 * Replacing the Nightingale Hood with a Shrouded Hood still gives the set effect. (Xbox360)
 * Can be duplicated in the Dawnstar sanctuary after buying the master bedroom upgrade from Devlin. Place the armour you want duplicated into the inventory of the manniquin then take it back again (without leaving the inventory menu). leave the area. Come back and the duplicated armour should be there (minus any improvents you have put unto it