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 in an effort to control them. They were supposed to be kept secret, but some were leaked out by one or more powers.

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.

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.

Neverwinter Nights: Hordes of the Underdark
In the computer game Neverwinter Nights: Hordes of the Underdark true names can be learned from a being known as the Knower of Names, who knows all of them but is limited in telling them by the gods' demanding a sizable monetary sacrifice from the asker for each name. Each individual has only one, and when it is spoken, is under the complete control of the speaker, directly compelled to do anything they order.

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.

The Icewind Dale Trilogy
In The Crystal Shard by R. A. Salvatore, it is implied that the true name of such a being as Errtu is the same as its given name. Drizzt Do'Urden draws Errtu's attention by speaking out loud this "true name".

In Streams of Silver, Dendybar the Mottled torments the spirit of Morkai the Red with the help of his true name in a manner similar to the 2nd Edition spells mentioned above, but it's not revealed how he learned this name and what it is.

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.