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)
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. This obscured any facial features that they may or may not have.
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)