The Ilwrath are spider-like beings
The Ilwrath were a scary and violent alien race even before their subjugation by the Ur-Quan Kzer-Za. As battle thralls, they now have access to Hierarchy resources and advanced technology, which they use to spread death, doom, and destruction. The Ilwrath's vile nature is deeply rooted in their entire civilization's zealous religious devotion to their evil gods.
The Ilwrath are large, red, spider-like aliens with numerous appendages (possessing a regenerative ability), many eyes, and pincer mandibles (with which they communicate). Like Earth spiders, they are born from egg sacs that are presumably filled with many eggs. Upon emerging from the eggs, the Ilwrath hatchlings attack and consume each other in a cannibalistic frenzy known as the Birth Rite.
As a species, they are psychopathically violent and religiously fanatical. The Ilwrath indeed describe themselves as being the very definition of "Evil", and to humans and many other sentient species they do appear to be almost demonic in their appearance, ways and customs.
Humans have many horror stories of the Ilwrath and the gruesome things they do to their prisoners of war, including dismemberment, evisceration, and other forms of torture meant to please Dogar and Kazon-for a good reason. After the Ur-Quan Hierarchy's victory in the Ur-Quan Slave War, the Ilwrath planned to sacrifice all Human beings to their dark gods in a bloody ceremony, but the Ur-Quan Kzer-Za did not allow this, as it conflicted with the Path of Now and Forever.
The Ilwrath pilot the Avenger-class ships.
The Pkunk tell that the Ilwrath were once a species of supreme and absolute good, but they tried to become even more good, which caused them to "wrap around" the good-evil spectrum, resulting in the pure evil species now associated with the name Ilwrath. The accuracy of this tale is uncertain, due to the playful nature of the informants.
Details about the Ilwrath prior to their supposed transformation are unknown.
Early in their history, the Ilwrath had a subsistence hunter-gatherer society and worshiped many gods, from Zith of the Pelt to Awk of the Seds. We know little of early Ilwrath history as the Ilwrath refer to these as the "Dark Ages" of religous confusion.
At a point in their history, a consolidation of the existing Ilwrath religions took place. A priestly ruling body deliberated on the multitude of belief systems and concluded that the Ilwrath must worship the twin gods of Death Dogar and Kazon, and discard all other gods "be they of Hearth or Flowing Web." The new ruling class declared that only priests are capable of interpreting the will of the gods, and refined the techniques of worship to perfection. Specific examples of doctrine included the eating of Heretics and the surrender of all possessions to holy sites or priestly dwellings.
After this point, Ilwrath society focused itself entirely on satisfying the will of the twin gods. Ilwrath technology advanced towards the creation of more efficient means of murder. Religious rituals revolved around pain and death, as is exemplified by a common act of affirmation that involves full or partial amputation of one's own limbs. Even the many deaths at the Birth Rite was interpreted as an Ilwrath's first tribute to the twin gods.
Prior to their contact with the Ur-Quan, the Ilwrath had probably developed rudimentary space-faring technology. The Chenjesu and Mmrnmhrm reported contact with the Ilwrath, but considered them a negligible threat at the time due to their primitive technology.
Subjugation by the Ur-Quan
The Ilwrath were among the first races enslaved by the Ur-Quan, a particularly simple task given their extremely low technological level. Seeing the potential of the violent and fanatical Ilwrath as Battle Thralls, the Ur-Quan gave the Ilwrath the technology to improve their ships to match that of the Chenjesu and the Mmrnmhrm. The large number of Ilwrath ships as well as the Ilwrath's religious zeal in seeking the death of other races proved to be an asset to the Ur-Quan offensive.
Upon the defeat of the Humans, the Ilwrath prepared a grand ceremony to murder all of the Humans, which was to be called "Mountains of Flesh". As the Humans had elected to be slave-shielded, the Ur-Quan stopped the Ilwrath ceremony, forcing the Ilwrath to use species indigenous to their homeworld for their sacrifices.
Some years after the end of the war, a bureaucratic error resulted in the depletion of the supply of creatures for sacrifice on the Ilwrath homeworld. A great crisis occured in Ilwrath religion and appeals to the twin gods for guidance became much more frequent.
The calls for guidance were answered. Around 2147, a hyperspace broadcast, claiming to be from the gods Dogar and Kazon, was heard on Hyperwave Channel 44. The broadcast ordered the Ilwrath to attack the "Bird Beings". The priest ruling class, feeling a threat to their authority, counseled the general population to be skeptical of the broadcast. However, the lack of sacrificial rituals, compounded with over-taxation, lack of "quality death in ceremony" and the over-complicated pronouncements of the Dark gods had apparently produced a religious void in many of the Ilwrath, prompting the majority to murder the entire priest caste. After this casual slaughter, the Ilwrath then embarked on a campaign of genocide against the Pkunk, devoting almost their entire fleet on the destruction of the Yehat sub-species.
The transmission that incited the Ilwrath to attack the Pkunk did in fact not originate from Dogar and Kazon. It was the Umgah, having built a powerful HyperWave Broadcaster, who were executing a practical joke. The "divine" order to attack the "bird beings" could however equally apply to the Yehat. The actual intent of the Umgah is unknown.
Relations with other races
The Ilwrath are Battle Thralls of the Ur-Quan Hierarchy. Due to their zeal in battling other species, they are the backbone of the Hierarchy offensive. Along with the Spathi, the Ilwrath were commanded to enforce the restrictions placed on Earth, but around the year 2137 abandoned their post when their Gods supposedly ordered them to "kill or at least torture someone soon," as Fwiffo puts it.
While the Pkunk bear no ill will towards the Ilwrath, the converse cannot be said. As with many, if not all, living things, the Ilwrath consider the Pkunk merely as a source for their cruelty and killings. Due to the Umgah deception, it is specifically this species that the Ilwrath set their sights on. In fact, their intention was a complete genocide. However, after several years of slaughtering Pkunk, the Ilwrath welcome the opportunity to direct their violence towards a less "pitiful, easilly-killed" species.
Near the end of the Second War, The Captain has the chance to manipulate the Ilwrath by using a Hyperwave Broadcaster to order them to abandon their jihad against the Pkunk. This would have the result that the Ilwrath go to battle the Thraddash instead. The Ilwrath would lose two Avengers for every one Thraddash Torch that they destroy, but since their fleet is roughly twice the size of the Thraddash fleet, this battle could lead to the mutual destruction of both fleets unless The Captain ends the Second War before they annihilate one another.
Sources of information
In 2155 the Ilwrath Sphere of Influence is not centered on their homeworld. However, the Ilwrath homeworld is near the center of their sphere as shown on the HyperSpace map from 2133. Any Ilwrath patrol can provide the coordinates of their home star system when asked about "Doggone" and "Quasar".
Both the Pkunk and The Safe Ones allude to the Umgah impersonation of Dogar and Kazon. The Umgah themselves and the Melnorme are much clearer, explicitly stating the extent of the prank and the use of the Umgah HyperWave Broadcaster.