First Restoration of D'ni
In 9463 DE, Atrus and Catherine visited the Age of Averone. There, they enlisted the help of its inhabitants to excavate the collapsed exit from Atrus's prison at the base of K'veer—an island in the D'ni cavern. For four years, they worked to clear rubble and dig out of the room. Finally, in 9467 DE, Atrus and his team broke through to the other side, unlocking his access to D'ni for the first time in over 30 years.
While this was supposed to be the end of Atrus's relationship with the Averonese people, the island's elders concluded that they could not expect the younger generation to go back to the way things once were. Many of the island's young adults joined the effort to restore D'ni, on the condition that they return to Averone to share what they had learned.
The Book of D'ni
The events of the First Restoration are chronicled in the novel Myst: The Book of D'ni, which was adapted from journals left behind by Catherine for others to discover.
Over the year or so that followed Atrus's breakthrough back into D'ni in 9467 DE, he, Catherine, and the Averonese worked to collect as many Books as they could find, hoping to come across survivors of D'ni's fall. While many of the Ages they found had been contaminated by the plague unleashed by Veovis and A'gaeris, they eventually uncovered over 1800 D'ni who agreed to return and help rebuild. Many were former members of the Guilds, and others—like Oma and Esel—grew up learning from their relatives and teaching themselves.
In 9468 DE, while working in the lower levels of the Guild Hall, a team uncovered an ancient passageway that had been built over by later generations of D'ni. The passage led to the Tomb of the Great King. Within it, they found an ancient Book which linked to Terahnee, where the bulk of the Ronay had fled the destruction of their home world Garternay—a home they shared with the D'ni over 9,400 years ago. With great trepidation, Atrus and Catherine decided to take a team to explore Terahnee, hoping to learn more about their shared history. Instead, because the Terahnee lacked an immunity to a common D'ni stomach bacterium, they unintentionally unleashed a disease that ravaged the Age's population, unveiling the dark underbelly of chattel slavery that maintained the Terahnee civilization in the process.
When Atrus and his team returned to D'ni from Terahnee, he came back with a determination to give the D'ni survivors a fresh start, free from what he called "the tragedies of their past". He saw the cavern as too tainted by the history of prejudice and greed to be able to continue rebuilding there. With the support of Catherine and the rest of the D'ni, he wrote a new Age, Releeshahn, where they could make that fresh start. The cavern itself was abandoned completely in 9470 DE when the remaining D'ni relocated to Releeshahn, while Atrus and Catherine moved to Tomahna.
When humans from Earth's surface rediscovered D'ni in the 1980s CE (9625s DE), they came across the journals left behind by Catherine which recounted the story of Atrus's life, including his attempts to rebuild D'ni. These journals were adapted into the games Myst and Riven, as well as the novels Myst: The Book of Atrus, Myst: The Book of Ti'ana, and Myst: The Book of D'ni.
During the First Restoration, the following Ages were recovered by Atrus and his companions. None of these Ages are currently known to be in the DRC's possession, and their whereabouts are unknown.