The Relyimah were slaves of the Terahnee, forcibly taken from their home Ages at a young age and stripped of their humanity in underground training centers. The word "relyimah" literally means "the unseen" in D'ni and Terahnee, and the Relyimah were expected to be silent and invisible. They lived in despicable conditions, crammed into small living spaces, and were forced to be silent at all times. The Terahnee, meanwhile, were conditioned not to see the Relyimah, and trained not to acknowledge their presence. Any slave who so much as touched a Terahnee was killed for such audacity.
While this slave trade was authorized and overseen by the king of Terahnee himself, the responsibility of training, overseeing, and managing the slave population was delegated to a race called the P'aarli. The P'aarli were, essentially, the ones doing the dirty work, while the Terahnee remained detached from the gruesome barbarism that supported their society.
When Atrus and his companions rediscovered Terahnee during their attempts to restore the D'ni cavern, they brought with them bacteria which proved to be fatal to the Terahnee, the P'aarli, and the Relyimah alike. While the Terahnee were all but wiped out by this plague—dropping from 200 million to less than 100,000 in only a few weeks—the Relyimah existed in such numbers that even after their considerable fatalities, they still numbered almost 2 billion.
After the worst of the plague had passed, the resulting power vacuum led to a series of conflicts between two main factions of the Relyimah. One, led by prominent Relyimah figures Hersha, Baddu, and Gat, was supported by Atrus and Eedrah Ro'Jethhe, a Relyimah sympathizer. The other was led by a Relyimah named Ymur, who was driven by blind rage toward his former masters and refused to accept the guidance or leadership of anyone but himself. Ymur eventually declared war on his fellows, but was killed before the ensuing battle by a young boy named Uta.
With Ymur gone, the rest of his army fled, resulting in a peaceful resolution to his unexpected uprising. Afterward, leaders like Hersha undertook the arduous task of building a new society for their kin, aided by a set of ancient D'ni laws which Atrus brought from D'ni for their reference. When Atrus and his companions departed, the temple containing the link back to D'ni was sealed once again, leaving them to their own fate on the Age that they had re-christened "Devokan"—"hope".