As a preface, I am looking at using some Astartes techmarines in-training as part of my Mechanicus army. The idea being that they were present on Forgeworlds undergoing their initiations and training with the Omnissiah's secrets when the Heresy unfolded. Ideally they weren't immediately recalled to their respective legions, but rather caught up in the mobilisations for war and some remained attached to their assigned mechanicus factions. With techmarines being supposedly held with suspision by their brethren, I also felt that maybe some of them felt more at home going to war with the Mechanicum - perhaps the awareness that their legion was purging any loyalist astartes from the ranks would rouse their caution in returning.

Does this sound reasonable?

Yeah I can imagine something like that happening - there were a fair number of Astartes dropped on Istvaan to be firebombed by Horus, and they were marines in direct contact with the legion, so anyone further away from the influence of Horus at the time you have to say might go through the same dilemma.

Like you say Techpriests are pretty estranged from their bretheren in any case, and provided they were with the Mechanicum who didn't side with Horus then I can definately imagine something like that happening!

Is this a new force you're planning BTW mate? :)

It's actually for my Dark Mechanicus :)

I have a number of marines I built up to be 'Techies' and adepts as part of the force, so am exploring options for them. Perhaps proto-obliterators, giving some potential to further modeling ideas.

Certainly I would imagine there would be both full and trainee Techmarines from the Traitor Legions that would have been on Mars when the Heresy broke out, and it would make sense for some of them to throw their support behind the Traitor Mechanicus rather than attempt to return to their Legion.

On the other hand, I suspect that 'proto-obliterators' might be taking it a bit far. The Inquisitor article on the Obliterator Virus makes it quite clear that the virus developed in the Eye of Terra at some point after the Heresy.

Just give them a souped-up weapons harness with ridiculous guns and you're probably ready to go... :)

Excellent.

I'm also wondering whether some would hold stronger fealty for the Mechanicum than their own legion. ie, how likely would an Imperial Fist techmarine based on Mars fight alongside Kelbor Hal during the Heresy?

If they were devoted disciples of his, then yes? All Techmarines are somewhat distrusted or separate from their legion brothers, IIRC.

Justify it by saying the Techmarine got lobotomised by rogue scrapcode during the uprising!