The D'myurj were a powerful and technologically advanced race, first encountered by the Federation in the late 24th century, although they claimed to have been intervening in Federation affairs much longer. They employed the Brethren as foot soldiers. (NF novel: Treason)
Since they came from a pocket universe, they could only exist in the primary universe for a few months at a time, and then the cellular degeneration would kill them. For this reason they used the Brethren as their soldiers and operatives. (NF novel: The Returned, Part 2)
They had translucent bodies that constantly shifted and shimmered, and were suffused with a glowing blue light that seemed to come from everywhere and no where at once. Instead of a nose they had a slit in the middle of their faces. Their heads had a triangular shape. They had no hair, and had large blue eyes with no pupils that almost took over half of their face. (NF novel: The Returned, Part 2)
As a culture they believed that they were helping to advance other societies, helping various races reach the pinnacle of their genetic development and eventually becoming beings of pure thought, like the Organians. Such beings do not pose any threat to the universe because they were part of it, not like corporeal beings whose actions could have terrible consequences.
The D'myurj believed that it was the destiny of all races to go on this path sooner or later, but they did not rush the process as they thought there was still time before anything extremely bad happened to the universe. However, one radical known as the Visionary did not want to wait, believing too much damage was happening to the universe and decided to destroy life with the help of the Brethren. (NF novel: Blind Man's Bluff)
On AF1963 they "grew" mindless bodies to inhabit and infiltrate all aspects of Federation life. Selar destroyed the operation by setting her phaser to self destruct, at the cost of her own life. (NF novel: Treason)
The D'myurj were all exterminated (except for two individuals) by the Dayans in 2380. Mackenzie Calhoun found this ironic since it was the same fate that they reserved for his people, the Xenexians, some months before. (NF novel: The Returned, Part 3)