True name

A true name was a creature's hidden name that, rather than just acting as a label, metaphysically truly belonged to that creature. All beings had true names, though most of these were unknown, especially to mere mortals. Knowing a creature's true name could grant power over it. There were several different accounts of true names and how they functioned in this sense.

Description
True names were given to all creatures by an agreement between the powers (that is, gods) in an effort to control them. They were supposed to be kept secret, but some were leaked out by one or more powers. True names could be learned from a being known as the Knower of Names in the layer of Cania in the Nine Hells, who knew all of them but was limited in telling them by the gods' demanding a sizable monetary sacrifice from the asker for each name.

The gods themselves apparently did not have true names. However, the only entity that knew the true name of the Red Knight, Faerûnian goddess of battle tactics, was Tempus, whom she saw as a father figure. If anyone were to learn her true name, they would be privy to every stratagem and battle plan on Toril.

For some planar beings such as Errtu, it seemed that their true name was the same as their common name.

Uses
When a creature's true name was known, a non-chaotic spellcaster could use it to cast spells such as true name, cacofiend, ensnarement, and spirit wrack, which harmed, bound, or controlled the creature. It could also be used to bind a summoned fiend to make a pact it would be compelled to keep.

Each creature had a unique true name among the Words of Creation, a language so primal even speaking a word of it had a powerful effect. A character who had knowledge of this language could learn a creature's true name and use it to weaken the creature's resistance against different effects or to teleport it.

It was even possible that when an individual's true name was spoken, they were under the complete control of the speaker, directly compelled to do anything they ordered.

Notable Uses
In 1356 DR, Drizzt Do'Urden was able to draw Errtu's attention by speaking out loud his "true name".

That same year, Dendybar the Mottled tormented the spirit of Morkai the Red with the help of his true name.

Neverwinter Nights 2
In Act II of the computer game Neverwinter Nights 2, five Luskan mages led by Black Garius are slain by the PC, but are then resurrected by the King of Shadows in the form of shadow reavers, near-invulnerable aberrations under the King's direct control. Luskan's rulers, the Arcane Brotherhood, see this as a potential political quagmire and begin expunging records related to Garius and his cohorts. These records include the true names of the mages: in order to keep control of their members, the Brotherhood always collect their true names as a matter of course. Sydney Natale, however, after learning of the reavers, holds back the names of Garius and the others from the records, and performs a divination upon them which reveals the true names of the reavers. The PC obtains the list of names following an unsuccessful attempt by Natale to double-cross the player, which results in the Luskan mage's death. Speaking a shadow reaver's true name in its presence strips it of its invulnerability, allowing it to be destroyed.

In Ammon Jerro's haven, The succubus Blooden will sometimes offer to tell the PC the True Name of Zaxis the hezrou, claiming that it will please her if the PC uses it to torment Zaxis. The PC can then order Zaxis to open the portal to Jerro's quarters, or to stop speaking about itself in the third person (unfortunately, Zaxis is too stupid to comprehend this order). The True name can also be offered to Hezebel in order to have her open the portal.

Neverwinter Nights: Hordes of the Underdark
In Neverwinter Nights: Hordes of the Underdark, the Knower of Names can tell you the true names of your allies, enemies, and even you.

True name of the PC
Your true name is based on your alignment.