Language of death

The language of death is a secret language known to all Scourge and Cult of the Damned members. To outsiders, its existence was never proven until the war against the Lich King, where forces of the Alliance and Horde found Flesh-bound Tomes written in blood and using the language. They were translated in order to decypher the Scourge plans in the Dragonblight.

Speculation

 * The names of the great gates of Icecrown—Aldur'thar (the Desolation Gate), Angrathar (the Wrath Gate), Corp'rethar (the Horror Gate), and Mord'rethar (the Death Gate)—are possibly taken from the language of death, since they do not seem to match any other known language.
 * The name "Mord'rethar" resembles both Mort'regar (a wing of Torghast) and Mordretha (a Necrolord champion); as such, the language of death might originate from the Shadowlands.