A pair of massive sa'angreal created at the end of the War of the Shadow by Aes Sedai looking to defeat the Dark One's forces. Each of the sa'angreal is in the form of a massive statue - one of a man, one of a woman, both said to look wise and serene. The male sa'angreal is buried near Cairhein and is being dug up now, whereas the female sa'angreal is on the island of Trelmaking in the Aryth Ocean.
The Choidan Kal are by far the most powerful sa'angreal ever made, so powerful in fact that they required special access ter'angreal to be used. Many in the Age of Legends thought they were too powerful to be used, that they would shatter the world.
The reason the Choidan Kal was never used is due to a split in the Hall of the Servants. One faction, led by Lews Therin Telamon, wanted to strike at the Bore and seal it using seven "focus points" created from cuendillar. The other faction, led by a female Aes Sedai called Latra Posae Decume, believed that using the Choidan Kal was the better solution. She managed to get the support of every female Aes Sedai in what was called the "Fateful Concord", thus stopping Lews Therin's plan, but before the Choidan Kal could be used the access ter'angreal were lost, and never recovered.