Terahnee is an Age which was written by the Ronay people of Garternay as a new home, when Garternay became unable to sustain life. The majority of the Ronay people left for Terahnee, only a small, isolated fraction led by Ri'neref left for D'ni instead.
Unlike D'ni, Terahnee was an immense green world, many times larger than Earth. The Ronay exiles, calling themselves now Terahnee, remained a proud and somewhat cruel people. Like the D'ni, they could Write, but like Atrus' father Gehn, they believed themselves to be better than the peoples from their Books (ahrotantee, Book-worlders), whom they treated as servants and even slaves.
The P'aarli people were trained as stewards and slave-keepers, and using them as a police force the Terahnee set up a society where they lived in unbridled luxury, while the Paarli and their slaves the Relyimah (lit. "unseen") lived in absolute poverty and servitude (it is said that the writers based the D'ni name Terahnee on the Greek word Tyranny).
As recounted in Myst: The Book of D'ni, this all changed when Atrus found a Link to Terahnee. His opposing views to those of the Terahnee led to a revolt under the Relyimah, but this was not in itself enough to cause a change. Unknown to all, over the millennia of separation there had been microbiological changes, and Atrus unwittingly caused a plague which nearly wiped out the Terahnee. Although the Relyimah also suffered grievous losses, after the plague subsided they by far outnumbered the Terahnee, and they rebelled, killing those few who survived.
Atrus mediated in forming a truce, but the end result was that the Terahnee had been wiped out: before Atrus' arrival there had been a population of over 200 million Terahnee and an uncounted number of Paarli and Relyimah, after the plague there were fewer than a hundred thousand Terahnee, while the Relyimah still numbered in excess of two billion. The Age was renamed Devokan, the D'ni word for "hope" and the linking books were permanently hidden, breaking the "link."