Not sure if I have the name right - it's either a displacement field or a displacer field.

This isn't the armour-energy-field thing, but something that's popped up in some Horus Heresy short stories - one used by the Custodes in "blood games" and by an ultramarine in the Iron Warriors short story with the upside-down fortress.

Essentially, it shrinks you down to disguise the bulk of a post-human, which I figure opens up a lot of options for nefarious doings in deathwatch missions.

I'm trying to find out if there are any more mentions of this device, especially its limitations, with the ultimate aim of writing house rules for it.

The things I can think of being a problem:

1) Astartes are supposed to have a distinct flat-featured look which Custodes may not. Not everyone would recognise it, of course, but particularly educated opponents might.

2) There would be a distinctive way astartes would move, even out of armour, because they never flex their back between shoulder blades and the small of their back due to the whole "fuzed rib-cage" thing

3) Not sure how well it holds up under physical contact - I can't remember if the custodes in blood games actually shake hands with anyone, for example.

2) There would be a distinctive way astartes would move, even out of armour, because they never flex their back between shoulder blades and the small of their back due to the whole "fuzed rib-cage" thing

There is no "fuzed rib-cage" thing.

I challenge you to find 1 reference of "fuzed rib-cage" in ANY bit of background. You will fail, because it does not exist.

The actual description is: "... solid mass of interlaced bone plates." (Index Astartes page 3)

The ribs of a space marine can still move, expand, and shift over themselves. If they could not the marine would die, as he could no longer work his lungs and would asphixiate. fuzed ribs would also destroy a majority of the muscles in the chest cavity when they get cut in half by the fuzing bones, something which would be quite painful and counterproductive.

There are some references of particular ways in which persons who spend a lot of time in power armor move though. One such reference is from a Gaunt's Ghost book where Gaunt is speaking to an unarmored Adeptus Soriatas and he notices and ponders upon the very obvious manner in which the sister turned her head. He mentioned it looked as if she was still wearing a helmet, and when she turned to look at him he couldn't help but think he was just target-locked. Space Marines would have similar traits, since they are almost always in their power armor.

As for flat faced, this would be a result of the bones of the head expanding from the changes wrought by the ossmodula. But the skull isn't as strongly affected as others, and the amount of flat-facedness is largely dependent on the chapter. The Lunar Wolves in the first Horus Heresy books were thus described but it was mentioned it was more of a result of their genetic heritige, how Horus had the same wide flat face and his sons took on those features through his gene-seed.