The Seven Devils
First, they argued over the best way to contain the captured demons, and each created a Hell with their own ideas of a prison, before The Nameless One decreed that the demons would be contained in The Abyss.
An age passed, and the arguments between the siblings grew worse. Eventually, the Seven began to scheme against each other. Soon, the schemes turned to skirmishes. the Elemental Titans tried to intervene, first diplomatically, then martially, and The Elemental War began.
The war shattered the Perfect World into pieces, and the Elemental Titans surrendered in order to keep the rest of The Ten Thousand Wonders intact. The Nameless One left Creation in disgust at the actions of his grandchildren, and The Seven were forced to rebuild the Perfect World from the pieces. As with the creation of the hells, the Seven could not agree on how to recreate the world, so they instead created three worlds instead: The World, The Feywild, and The Shadowfell. The Seven imprisoned the Elemental Titans within The Howling Storm, then settled in to rule over The Three Worlds.
They began to openly show cruelty and decadence during this period, which became known as The Age of Darkness. They ruled the mortal races with absolute tyranny, and imprisoned the souls of those who displeased them in the Hells they had once made to imprison demons. This practice trapped souls once destined to refill the Rivers of Time and Life, and threatened to drain them if not stopped. The Firstborn Gods quarreled over what to do with their wayward children. Eventually Kronus The Firstborn Son decided to act, in defiance of his wife, and showed nine mortals how to ascend to godhood. The Nine defeated the Seven, then opened the Seven Hells and led the trapped souls within to the River of Time. Then they imprisoned the Seven, each one in the particular hell that they had created.
The Seven Devils remain trapped in their hells, working through those they empower to break the seals of their hells, escape, and destroy the order of the Gods.