Thexian trade empire

[Bannik]

So what do we think fellas a threat or simply 40k ferengi (any info on them would be great)

[Ordo Hydra]

Sadly, there is no more information on them beyond that brief mention. I think mostly that they are a Tau sized menace though my impression of them is that they are content to remain in the shadows and simply infiltrate their enemies. A curious thing though is that their faction is called a "trade empire" suggesting that they make use of commerce and want profit.

[Lord Damocles]

Given what little we know of them:

'The Bloodmoons of Thex Prime
(approx. 15% of total Thexian Trade Empire)
Hailing from the oxidised worlds known as the Bloodmoons, the Thexian Elite are a biomorphic race few in number, but strong in influence. They are masters of manipulation and have worked themselves into integral positions in the Borlac, Loxatl and Nicassar civilisations. Though Thexians are famously persuasive, when guile is not enough their battle form is quite horryfying to behold.'
Rulebook (5th ed.), pg.117

They may be Vampires:

'Vampires
The ancient legends of Old Earth were for long ages discredited as invention, folk-lore and myth. The reality however, was to prove far more disturbing. Vampires are polymorthic entities able to change their metabolism in order to resemble the creatures amongst whom they live. Their natural shape (if such it can be called) is bat-like, although approximately human-sized. Vampires exist within the societies of most established intelligent races, and often assume positions of authority. They naturally crave for power amongst the intelligent races, regarding even fellow Vampires as threatening rivals. They have no home world. Where they evolved, and why they have come to occupy a parasitic position within alien species is unknown.
Vampires live amongst their chosen race after the manner of that race, and cannot be easily singled out. They have psychic powers of a level comparable to humans. Vampires maintain their lives by absorbing the life-force of other creatures, achieved by prolonged physical contact (a simple combat blow will not surfice).'
Rogue Trader Bestiary

[Ordo Hydra]

I bow to Lord Damocles since he basically provided everything we know about the Thexians. We really should call these snippets "Index Damocles" But yeah, I remember some people commenting that the Thexians might be vampires. I mean, some of the elements are there namely having a much more terrifying secondary form and their desire to infiltrate other civilizations. Plus, their homeworld has the interestingly named Bloodmoons.

Though this might be me misremembering it but I always thought the Rogue Trader vampires were warp entities rather than xenos of the material realm.

[Lord Damocles]

Vampires came under the heading 'Warp Creatures' (along with Enslavers, Astral Hounds, Astral Spectres, Psychneunein, Warp Entities and Zombies)


On a tangential note:
The illustartion of a Vampire (attached) from Rogue Trader looks very like the only description of a Cythor Fiend I've read -

Cythor Fiends were a race of extraordinarily evil aliens who lived in a desolate region in the north eastern corner of the Segmentum Obscurus known as the Ghoul Stars. High Marshall Helbrecht of the Black Templars Space Marines led a crusade against them a few years prior to the Third War for Armageddon. The few surviving records of the crusade describe the Cythor Fiends as humanoid Bat-creatures with leathery wings attached to their arms, which were topped with a long, curved claw with which to eviscerate enemies. They had bulging ice-blue eyes and many sharp shard-like teeth sticking out of their leering jaws. They bred in vast numbers and had a preference for swarming the enemy with massive wave attacks. Little is known about the Cythor Fiends' society or way of life, but Imperial Intelligence suggests that their civilization was based on the principles of Warlordism and "Rule of the Shrewdest". Fierce warlords commanded millions of bloodthirsty warriors armed with spike-shooting carbines and their wing-talons. Helbrecht lead the crusade to their homeworlds, but once they arrived found them strangely empty...Codex Black Templars pg44, other references?

Which would obviously provide a possible link between the Thexian Elite and Cythorians.
The Bloodmoons are close to the Ghoul Stars where the Cythor Fiends had their empire until Helbrecht of the Black Templars wrecked it up (Codex: Black Templars, pg.44); but I can't find any source which actually gives anything close to the physical description of the Fiends above.

[Idaan]

The RT vampire reappears in the new 6e Rulebook of course.

[Ordo Hydra]

No entry on them but yeah there is the distinctive vampire-ish appearance to them in the new rulebook picture. I do wonder also on the Cythor Fiends and Thexian connection. The Blood Moons are close to the Ghoul Region and we do have that mystery of the Cythor Fiends disappearing before the Black Templars came to wipe them out. Speculation on my part but I could very easily see the Cythor Fiends being footsoldiers for the Thexian Trade Empire.