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.

Knowledge
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. The Red Knight, Faerûnian goddess of battle tactics, was formerly mortal and hence had a true name. However, the only entity that knew her true name 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 truename, 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 fell under the complete control of the speaker, directly compelled to do anything they ordered.

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

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

In 1374 DR, five Luskan mages led by Black Garius were slain by the Kalach-Cha, but were 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, saw this as a potential political quagmire and began expunging records related to Garius and his cohorts. These records included the true names of the mages: in order to keep control of their members, the Brotherhood always collected their true names as a matter of course. Sydney Natale, however, after learning of the reavers, held back the names of Garius and the others from the records, and performed a divination upon them that revealed the true names of the reavers. The Kalach-Cha obtained the list of names following an unsuccessful attempt by Natale to double-cross them, which resulted in the mage's death. Speaking a shadow reaver's true name in its presence stripped it of its invulnerability, allowing it to be destroyed.

In Ammon Jerro's haven, the succubus Blooden sometimes offered to tell the Kalach-Cha the true name of Zaxis the hezrou, claiming that it would please her if they used it to torment Zaxis. The Kalach-Cha could then order Zaxis to open the portal to Jerro's quarters, or to stop speaking about itself in the third person (unfortunately, Zaxis was too stupid to comprehend this order). The true name could also be offered to Hezebel in order to have her open the portal.

Known True Names
Some known true names are as follows:

The true name of Drogan Droganson's Pupil depended on their alignment, as follows: