Atrus wrote a Descriptive Book for the Age of Averone. He and Catherine contacted the natives and arranged to get their help to break through from the room at the bottom of K'veer, where Gehn had imprisoned him, into the rest of the house, so they can access D'ni.
At first the elders of Averone made a deal with Atrus that after he had broken through, he'd leave Averone as he'd found it, never to return. After working with him for four years, and getting to know what an honorable person he is, they decide to keep the link open and to allow some of their young men and women to go with Atrus, provided they return to teach the younger ones at home on Averone.
So, a team of young people go with Atrus and Catherine to D'ni to search for Descriptive Books where survivors of The Fall might be living. They find 787 Books and gather more than 1800 survivors, who come to D'ni to work with Atrus to rebuild their culture.
They try to rebuild the City, but the going is very slow. The damage is monumental. In order to make a good start and to give the people hope and encouragement, they try to rebuild the least damaged of the Guild Houses, the old Ink-Makers Hall.
During their attempt, they find the damage worse than they had anticipated, but deep beneath the Hall they discover the long lost Temple of the Great King. Carefully, they break the seal on the door and enter the Temple. There are thousands of Descriptive Books within the Temple. These Books are thousands of years old and the Writing is slightly different from the D'ni characters they are used to, but they decipher it after some effort.
In the Temple, prominently displayed in it's own sealed chamber, is a large ornate Descriptive Book to an Age called Terahnee. Atrus and his friends decide to take a small exploration party there. They find a culture closely related to the D'ni. Apparently, the D'ni and the Terahnee all came from the same original home, Garternay.
The Terahnee have gone the way of Gehn on a vast scale. They've enslaved the people of certain Ages. They have servants of a different species known as the P'aarli, and these are the slave drivers. The slaves are taken as very little children and trained with torture so they will be obedient.
The slaves are "relyimah," the "unseen." They use passageways inside the walls and under the ground, and the Terahnee have been trained not to see them and to take their work for granted. They are trained to view the slavery of these people as being the proper way of things. Needless to say, these slaves have miserable lives. They have formed a loose sort of underground organization and hope to be able to win their freedom somehow.
The Terahnee welcome Atrus and his people, thinking they are all D'ni, but when they find out that Atrus is only ' D'ni, Catherine and some of the others are outsiders, and that Atrus insists that non-D'ni can Write Ages, they become furious. Atrus' thinking threatens the core of their warped society. They believe that only those of their blood can Write, and that this gives them the right to enslave the others.
Atrus and his party are detained until the King can send their death warrant. The King sends it, but by the time it gets to the house where they are being held, a terrible plague has fallen upon the Age. Almost all the Terahnee and the P'aarli are dying, but the slaves are surviving at a much better rate.
Soon the slaves rebel against their sick masters and take over the Age. There are battles fought against the slave-drivers, then a battle fought against one of the slaves who decided that he'd like to rule the others as the Terahnee ruled him.
In the end there aren't many Terahnee left alive. Their civilization is decimated. The P'aarli are vanquished. The slaves start governing themselves with a basic set of laws given to them by Atrus, based on the D'ni laws. Atrus and his group seal up the Book of Terahnee again, so the link between the worlds can't be accessed anymore.
The D'ni survivors, with some people from Averone and Terahnee, make a fresh start on a beautiful new Age which Atrus has written, instead of trying to rebuild in the D'ni Cavern. Atrus writes the Age of Tomahna, where he and Catherine live.