True name

A creature's True Name or true name is its hidden name that, rather just acting as a label, metaphysically truly belongs to that creature. All beings have true names, though most of these are unknown, especially to mere mortals. Knowing a creature's true name can grant power over it. There are several different accounts of true names and how they function in this sense.

The Red knight knows the true name of Tempus.

2nd Edition (Planewalker's Handbook)
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. When a creature's true name is known, a nonchaotic spellcaster may use it to cast spells such as true name, cacofiend, ensnarement and spirit wrack, which harm, bind or control the creature. It can also be used to bind a summoned fiend to make a pact it will be compelled to keep.

The powers themselves apparently do not have true names.

3rd Edition (Book of Exalted Deeds)
Each creature has a unique true name among the Words of Creation, a language so primal even speaking a word of it has a powerful effect. A character who has knowledge of this language can 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 KoS' 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 and enables 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 learnt this name and what it is.

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

Your true name is based on your alignment.

Lawful Good: Donita'ar the Blessed

Lawful Neutral: Lysisius the Firm

Lawful Evil: Dretchen the Dark

Neutral Good: Ebela'ar the Kind

True Neutral: Neuthroxus the Fair

Neutral Evil: Nel'zeben the Selfish

Chaotic Good: Kagita'ar the Heartseeker

Chaotic Neutral: Murdus the Wild

Chaotic Evil: Ba'elzaden the Cruel

The true names of your henchmen:
 * Valen Shadowbreath: Oeskathine the Demonwrestler

The true name of the Knower of Names: Pyreshi the Knower

The true name of Mephistopheles: Tra'axfyl the Ambitious

The true name of Cassandra: Cassanduria the Beautiful