Many among the watch fortress' leadership believe that Aun'La may in fact not be an Ethereal at all, but an attempt at misdirection and misinformation by the T'au in the Reach.
Aun'La was captured by a Deathwatch Kill-team on a T'au courier craft in transit. In an exceptionally difficult operation undertaken by Battle-Brother Pellas, the Gladius-class Frigate Thunder's Word slipped deep into T'au-controlled space and intercepted the Aun's craft as it passed through an uninhabited star system.
Precision weapons fire crippled the smaller ship's drives and a boarding party easily overpowered the xenos inside. The courier craft was rigged to suffer a catastrophic drive failure and the Thunder's Word disappeared into the void without sighting any other T'au ships.
As far as is known, the T'au are unaware that the Aun has been captured and believe him to have been killed in the accidental explosion of his ship.
Some in the Ordo Xenos in the Jericho Reach have cast doubts on the operation, however. Everything went a little too smoothly for wary Inquisitors to entirely trust the prize that seems to have fallen into their laps.
From the incautious transmissions that first alerted the Deathwatch to the Ethereal's travel, to the route taken and the absence of T'au fleet units in the area when the capture attempt was made, they see a set-up.
The T'au are fanatically loyal to their ruling caste and to put even such a low-ranking Ethereal at risk seems an act of uncharacteristic negligence.
Sensorium and augur scanning of the Aun'La have revealed no notable deviations from standard T'au morphology. There are no signs of extra cranial nodes that might implicate psychic control, no pheromone glands, no sub-sonic augmentations.
The conclusion being drawn is that this supposed "Ethereal" is nothing of the sort and has been purposefully given up to misdirect the enemies of the T'au.
All xenos are routinely examined down to a molecular level for concealed weapons and explosives before being brought to Watch Fortress Erioch, and the Aun'La is thought to pose no threat at present.
Nevertheless he is carefully watched at all times and kept fully quarantined from the other T'au prisoners of Magenta sector while the watch fortress' governing Chamber of Vigilance ponders his fate.
Some are moving for a full vivisection to settle matters once and for all while others are content to watch and wait for any underlying T'au scheme to reveal itself. The matter stands unresolved.
- Deathwatch: Rites of Battle (RPG), pg. 240