Can anyone tell me what the prefix in the term Arco-Flagellant means ?

Probably from the latin arcus, meaning "arch" or "bow." I don't think it has a specific intended meaning in this case, but is more used to imply a technological/archaic/arcane connection; that is, this person is not only a flagellant, but they are a mechanized super-flagellant.

That said, this is all idle theorizing, not based in fact.

Hmm, that makes sense. I mean, they are turned in to combat-servitors of a kind.

Thx for the quick answer!

Where do Rogue Traders usually keep their Warrants of Trade? I know of one example -- Augustus Killian, from Rogue Trader: The Koronus Bestiary -- where it was carried on the RT's own person via a stasis cylinder (thus implying that it's an easily concealable man-portable document, as opposed to speculation that it's actually a huge tome of legal jargon regarding the RT's responsibilities), and there's also mention in the RT Core Rulebook that old dynasties of RTs typically keep them in a stasis vault somewhere safe (on Terra according to The Explorer's Handbook, though that brings up the question of how the warrant-holder proves his credentials to Imperial authorities), but other than that, nothing.

Nemesis goes for the huge tome line of thought. I think it could be rather dependant on which system. For example, in a primarily feral system it's likely... possibly... printed on vellum and then held in stasis. On developed systems, it is likely that it would be stored on less archaic data systems, like the one you mentioned.

In one of the Enforcer (one about rouge traders) I believe it, one of the original ones was a single sheet of paper that was stored in a stasis vault.

In the Of mars series his paper was framed and above a fireplace. (Although it was one of the newer ones, granted to him directly)

I don't get how the C'tan are weak to psychic energies and hate the warp so much. They don't have any presence in the warp so why doesn't this make them like blanks?

They are 4th dimensional beings, that exist in an extra dimension that is not the warp.

Significantly bigger than a Marine, as far as I've gathered;

'Talos regarded the vaguely humanoid shape as his sight resolved into an approximation of clarity. The Exalted wore the same relic armour so revered by the Atramentar (Terminator Armour) but... changed. Warped. Literally. Occasional flickers of warp lighting rippled across the surface of the armour. The witchlight gave off no illumination of its own.'

'The Exalted dragged itself closer to the table. A mild tremor shook through the chamber as the creature took a single step.'

'A portrait of too many fangs and acidic drool. Talos glared into his lord's black eyes, and the twisted face of a man he'd once admired.'

'The Exalted licked it's teeth. It still wore its armour for the most part, although the ceramite had long since become part of its altered flesh. The apothecarion was expansive, but the Exalted's nature forced it into an uncomfortable hunch to avoid scraping its horned helm across the ceiling... the Exalted stroked a taloned finger across a surgical table.'

'Its lips peeled back from its shark's teeth in a grin in didn't feel'

'Metal wrenched in protesting chorus with the harsh words. The Exalted's monstrous talon deformed the wall where the creature gripped too hard. With a grunt, it pulled its claw free.'

'The daemon heaved its exoskeletal bulk out of the throne, armour joints creaking'

'Vandred gestured to the hololithic, forcing himself not to be distracted by the clawed monstrosity that his right hand had become.'

That's all the description I can find from a skim through the books.

I thank you for the that answer!!