Named the Fifth Age by Gehn, he wrote the Age's book by composing fragments of text written by D'ni Masters. It was probably Gehn’s best age, as far as size, technology, and stability went. Even still, Gehn's scarcity of words resulted to several inherent flaws, as all his ages did, and was in a steady state of decline.
To prevent Gehn from continuing his disastrous activities and systematic destruction of his Ages, Atrus trapped Gehn there for 33 years, destroying all linking books, trapping Gehn from ever accessing D'ni. During their confrontation, Catherine (with the assistance of Anna, who probably helped her by avoiding contradictions) wrote alterations in the Riven Descriptive Book. Her bizarre, intuitive rule-breaking style caused Rivenese daggers mysteriously appear around the island and open lava fissures; including the enigmatic Star Fissure.
Atrus attempted to make some corrections to the Descriptive Book, slowing down its collapse, but the instability remained. Atrus built a crystal viewer in Rime to have glimpses to Riven and determine the status of Gehn.
During this part of history, the island split into five. Gehn restricted four of those to himself and to his personnel. The villagers were limited to one island. He stripped them of their former beauty and riddled them with his self-absorbed closely guarded constructions. The Moiety rarely visit them. Gehn used bridges to connect his islands, but later moved to single-seat “mag-lev” trams. Gehn used one island to make books and revive the Art. On another he built a temple that provides power throughout the age. On a third he constructed a scale topographical map of Riven, and the smallest he used as a prison. Gehn’s prison was actually where Riven’s great tree was once located. It had once towered over the island, but Gehn cut it down when Atrus trapped him, and built a prison out of the stump.
Atrus first traveled to Riven in Myst: The Book of Atrus, where he met and fell in love with Katran a Riven native, later known as Catherine. It is where he came to know Catherine and where Atrus and Catherine were supposed to get married.
Riven’s inhabitants split into two factions: those who followed Gehn and those who rebelled against them. Atrus wrote improvements into Riven, but he could only slow the imminent decay. The final collapse happened when a mysterious friend of Atrus trapped Gehn and freed Atrus’s wife Catherine. After all the people had been moved to Tay, the stranger opened the Star Fissure, which called Atrus, but triggered Riven’s end.
The Riven book can be seen at the beginning of Myst III: Exile in Atrus' study. The gateway image in the linking panel appears as though the entire age has fallen to utter chaos after the opening of the Star Fissure.
Once a single island, Riven became a cluster of five islands as a result of the Age's instability. Four of the five are connected using flexible magnetic levitation vehicles ('trams'); steam pipes also distribute energy between them. The fifth island, on which stood an enormous tree, has long since moved away from the others.
Gehn wrote in the Age all the materials necessary to the craft of making books (and probably every other Age he ever wrote). This included the aforementioned giant tree (for paper), and a species of scarab beetle (for ink).
Gehn noted that the element of "five" was to be found everywhere in he world (eg. its separation to five islands, a piece of wood forming a five-rayed "star" at its center). He believed that this reflected the D'ni preoccupation with the number, as he used pre-existing writings to compose the Age. He further considered it to be evidence that Riven (as every other Age) was indeed created by these writings, rather than being a pre-existing Age to which the writings linked to.
In the course of 30 years, tectonic plate shifting split the island of Riven into five distinct pieces, about a half a mile apart. Gehn claimed them (except the Jungle Island) as his exclusive domain, with his ministers and personal militia allowed on them.