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 learnt 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 sizeable 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.

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 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