The Chthons were the last of the Immortals created by A-dui. Having used materials from all parts of the surface world He chose to reach into the heart of the world and used the magma to shape the Chthons. The heat of these rocks caused pieces of his hands to be absorbed into the magma, giving the Chthons their great strength and power.
The Chthons did not have a definite, standard shape since they were formed from the magma and dead earth of the Underdark. During The Age of Paradise they made their homes in the caves and openings that riddled Mire-Brissay beneath its surface, though they did come out occasionally to interact with A-dui and the other Immortals. Their greatest pleasure was in bringing life into the Underdark and watching to see what survived and grew out of the chaos they instigated.
When the Firstborn appeared in the world the Chthons once again brought life to the Underdark. In this case they took the Demondim and introduced them to a life in the dark world below the surface. Thus was born the culture known today as the Demonic culture. During the subsequent ages, the Chthons also had interactions with the other Firstborn and persuaded some individuals to give up the light of the day.
During the War of the Immortals the Chthons played a major role in both the instigation and continuation of the hostilities. It was only with the Accord of Withdrawal that they ceased open combat and disappeared back to the realm of the Underdark. However when The Shattering broke the world apart into Fragments the realm of the Chthons was exposed. This has not led to the open appearance of the Chthons once again, but their influence is growing. What they hope or wish for is unclear to most, though many believe it is the destruction of all life.