The Courts of Chaos is the other "pole of reality." It is even older than Amber. Dworkin is said to have fled the Courts with the stolen the eye of the Serpent of Chaos which he then used to create Amber while all the forces of Chaos opposed him. In the early days, small forces from Chaos would periodically venture forth to Amber and attempt to destroy it, with little success.
More recently there was a massive war between Amber and Chaos in which the lords of Chaos attempted, with the aid of the traitor Brand, to destroy Amber and Amber's pattern. They came very close, but ultimately failed, surrendering to the forces of Amber on their very doorstep. The war -- now known as the Patternfall War -- was costly for both sides. Many in Chaos are said to have died during the fighting. Amber's own losses included King Oberon, Eric, Dierdre, and the traitor Brand.
At the end of the war, Random was chosen as the new King by the horn of the Unicorn, and a peace treaty was established between the Court of Amber and the Courts of Chaos, represented by their King, Staffan. The current King of Chaos is Staffan's son, Edward.