Not much is known about the Lord of Chaos, despite being the title of the sixth book in Robert Jordan's "The Wheel of Time" series. Some think that it may be another name for the Dark One, but it is unlikely. It seems more likely that the "Lord of Chaos" is an appellation referring to Rand al'Thor and his ta'veren influence. A command given to the Forsaken Demandred by the Dark One, "let the Lord of Chaos rule" suggests that it may only be a phrase directing the Forsaken, notably to spread disorder and distrust.
Other mentions of "Lord of Chaos"
The Fourth Age
The "Lord of Chaos" is mentioned in a children's game from the Fourth Age. It is represented in its entirety at the beginning of Lord of Chaos and seems to be the first time this phrase is mentioned in the series.
The moon by day, and the sun by night.
Blind woman, deaf man, jackdaw fool.
Let the Lord of Chaos rule.
——chant from a children's game
heard in Great Arvalon,
the Fourth Age
Of the Shadow
The Feast of Fools
A holiday called the Feast of Fools is described as:
—TWORJTWOT, Chapter 31
It is interesting to note that Rand has been given the title Lord of Chaos, and he certainly is a chosen one from a variety of prophecies. By merit of being ta'veren and a fulfillment of prophecy, a sheep herder with no experience or qualifications for ruling gives orders that are obeyed across nations.
Possibly, this connection to the Feast of Fools is the common association with the "Lord of Chaos," whether used by Darkfriends, in children's chanting, or in direct reference to Rand al'Thor.
Knife of Dreams - Prologue: Embers Falling on Dry Grass
A woman in roiling flames appears and says "I am Semirhage", she offers Suroth to rule in the lands of the Return, tells Suroth of the death of the Empress Radhanan (might she live forever) of Seanchan, suggesting that Suroth could name herself Empress.
Suroth fears the the punishment of having her skin removed alive, for usurping the throne.
Semirage says, “I see I didn’t make myself clear. Radhanan is dead, and her daughters, and her sons, and half the Imperial Court, as well. There is no Imperial family except for Tuon. There is no Empire. Seandar is in the hands of rioters and looters, and so are a dozen other cities. At least fifty nobles are contending for the throne."
Semirhage says that if Suroth disposes of Tuon, she is safe to declare herself Empress and tells her a ship from Seanchan will soon arrive with the news.
(Semirhage) She laughed again, and said something strange. “Let the lord of chaos rule.”