Originally Posted by Lord of the Night
...no definitive explanation is given for how they got aboard...
I thought as much. Allow me to provide some ways they got on board:
1) They snuck aboard by hiding in a craft leaving the dying world.
2) They were delivered via this spore mines they have.
3) They were already on board and the crew was under their control.
Now 3) is obviously not plausible, since I'm fairly certain that the Grey Knights would have noticed their crew acting strange. And if the Genestealers had the crew in service to them, they'd have waited until they got to a better target rather than butchering the crew and the Grey Knights.
Option 2) is slightly more plausible, but it makes no sense as to how the Genestealers got the drop on the Grey Knights, since the ship would have known it was hit by the spore mine and whatever delivered it (hive ship, etc) would have likely engaged the ship and the crew and the Grey Knights would have known that they were under attack, not ambushed while they were isolated and alone as the author maintains.
Finally we have option 1). Now if some unspecified vessel boards the Grey Knights ship, it either does it uninvited or invited. If it is uninvited then it's a threat and we have the same problems with option 2). If it was invited, we have to ask why. If it was a refugee ship from the dying planet, it never would have got within a million miles of the ship, because the Grey Knights work ceaselessly to preserve the secrecy of their existence. If is was an Astartes ship, they'd have been told to piss off. So the only real option is that it was a ship from the Inquisiton. But is it was an Inquisition ship why did the Grey Knights need to be there? And if they had been sent as back up, they'd have known the threat they faced and been on alert against Genestealers and other Tyranids, which the author maintains they weren't.
So the only option left to us is to assume that they made it back on a ship that left the Grey Knight's ship, presumably the Purifier's Storm Raven. That means that a bunch of trained Astartes managed to accidentally let on board a brood of 60 Genestealers. If that's the case then they deserved to die because they are fucking idiots.
Furthermore, if this brood of Genestealers is so powerful and their Shadow in the Warp so potent that it is able to overcome a being of legendary will power that even the Chaos Gods cannot overcome, how did the Grey Knights not sense it? Surely if this brood of Genestealers is so powerful at least one of these psychic individuals would have realised and warned his brothers.
I cannot conceive in my mind of any plausible way that this group of Grey Knights is 'ambushed' by a brood of Genestealers, that doesn't make them look like anything less than idiots. And I suspect that the author couldn't think of one either, which is why there isn't an explanation given. The author can't be fucked to come up with an explanation, the situation just arises because "fuck it, it'll be a larf". No explanation, no reason, nothing. Add that to the questionable idea that a Grey Knight can be psychically dominated and the brood of Genestealers which overcame the 10 Purifiers (and the chaos cults which took up subsequent residence) can't kill the similar sized force sent to investigate and you have everything you need to convince me that the author is just lazy and and makes up incoherent plot devices that are ill conceived and shoody in order to sell a 2-bit audio book that is inconsistent and the same crap that the Black Library seem to be churning out of late; a hastily written, poorly conceived and poorly managed piece of shit that is just intriguing to get people to by the day or two after its released and is immediately forgotten about just as nearly everyone realises its crap.