Note: Fictional details from the Myst franchise follow, and will be treated as facts.
A'Gaeris (pronounced ay-GAR-iss), was a pivotal character in the downfall of the civilization of the D'ni, introduced in .
A’Gaeris was one of the men responsible for the destruction of D’ni. He was embittered with the D’ni, who had expelled him from his Guild, accusing him of stealing a Book. Adding insult to an already unbearable injury, A'Gaeris lost his fiance in the scandal as well. Forced to live among the commoners of D’ni, A’Gaeris plotted ways to get back at the Guilds that had rejected him, and eventually plotted to destroy the entire city. He became well known among the lower class D'ni people as a writer of pamphlets aimed against the D'ni people, and he would sign these pamphlets simply as "The Philosopher."
He waited fifty years to make his move, for he had no influence in the city anymore. Finally, his chance came. Influenced by his assistant, Suahrnir, an influential council member named Veovis came to him. A’Gaeris was able to turn the D’ni Guilds against Veovis, and this act solidified Veovis’ alliance with A’Gaeris. Together they plotted to overthrow - if not destroy the heart of the civilization that had destroyed them. Before this could be started, however, Veovis was arrested for his deeds. A’Gaeris freed him, and they escalated their plans to include the wholesale destruction of D’ni. Together they released a plague into D’ni’s ventilation system, and then proceeded to infect almost all the Ages that the D’ni had fled into.
A’Gaeris then killed Veovis for refusing to write an Age in which they could be “gods” together. He then proceeded to capture Ti’ana and Gehn, Aitrus’ wife and son. Aitrus, dying of the plague that Veovis and A’Gaeris released on the city, was able to entrap A’Gaeris in a fiery Age, sacrificing his own life in the process, and finally bringing an end to this madman.