So it opens with the Retributors captain and sergeant on board the chapter battlebarge overseeing the boarding action by the Impulsors (the designation of this particular marine squad) led by sergeant Kohren. The astartes (possibly acting under the mandate of the Inquisition of visa-versa) have already gained possession of one sphere, we'll call this "Sphere 1".
Sphere one is held by psychic dampeners/some form of containment mechanism to be studied by the Inquisition.

On the enemy ship the impulsor squad moves into the chamber and trips some sort of alarm system. As they walk in further they see a half-constructed talos golem - presumably some sort of mechanized body intended to house the entity within "Sphere 2" (possibly their overall mission, to gain physical form). Surrounding this construct are many psychic amplification devices (spine and mask) which we saw the psykers using previously. This indicates either that they were being used to empower the golem which was requiring a lot of energy to create or that they were planning on possibly "upgrading" other suitable members of their rebellion to the status of psykers.

As the Astartes come face to face with Sphere 2 it attempts to ward them away or dominate their minds with wave after wave of psychic attacks which the marines shrug off. Sphere 2 then shows that it knows the marines by naming them "astartes" as though cursing. This suggests that the two factions have prior history with one another. Sphere 2 feigns weakness as the astartes begin installing what appears to be handheld psychic dampeners of containment devices.

As it cuts back to the battlebarge we see the Inquisitor dive into the warp and witness a communication between Sphere 1 and Sphere 2 which to my understanding goes as follows:

Sphere 2: Who's there?
Sphere 1: We have failed, the Astartes defy our touch. We must return, break your seal!
Sphere 2: We will never survive
Sphere 1: YOU MUST! (Survive/Break seal) take them with you ...

At this point the Inquisitor makes a mistake, he ejects himself from the conversation and lets down his guard to warn the captain saying "The astartes are in danger!". The captain conveys this to the tactical marine in the room who rushes to warn the squad.
This alerts the sphere in the room (Sphere 1) to his presence and attacks. We here him whisper "He knows" and the Inquisitor is forcibly jerked back into a trance-like state. This is followed an energy discharge that we see directly hit the inquisitor. At this point the psyker attempts to fight back by plunging once again into the warp

We are shown (what I believe) is a vision of the future, of what will come to pass. The psyker is overwhelmed by the sphere and is taken over completely at which point the marines put him down with prejudice (I especially like the little detail of the sergeants bolter detonations blowing back the captains cloak showing the sheer force of bolt rounds)

Back on the heretics ship we see sergeant Kohren receive the warning transmission. Unfortunately it is too late and the entity within Sphere 2 withdraws, drawing the astartes in in its wake. Here we see what i believe to be the creatures true form, a tentacled, gargoyle-like beast. It breaks down the seal (sphere) as part of its escape. The Astartes are sucked into the sphere alongside it, possibly as a side effect of the seal being broken, possibly by the creature itself's power, either way the result is the same.

We then see the creature in its true warp form, presumably translating through the warp and taking the astartes with it. In the warp-like nebulae we see clouds of black, these could translate to worlds, landmasses, daemonworlds, warp storms etc but a single black cloud rushes up to meet them consuming the creature. I interpret this as the creature selecting its homeworld and descending upon it with the result looking like it is coming up to meet them. However it could represent a pulse of energy from a 3rd party, maybe the final desperate last act of the Inquisitor?

Either way, sergeant Kohren is deposited in a strange land, a valley surrounded by what looks like the statues or mummified remains of ancient kings/xenos/daemons. We can't tell whether its a real planet, a daemon world or even real at all. Khoren vomits and the blood immediately coagulates (and we get a great face reveal). The video ends with the valley being put into perspective as the rest of Kohren's impulsor squad materialize on the surrounding pillars. It is revealed that Khoren himself is standing on a pillar himself which puts into perspective the enormous size of the surrounding objects


What i love especially about this series and why I love this part the most is its ramping feel. The process begins with the calm, silent, almost serene boarding action as the assault craft drifts through space towards the heretics ship. The tension builds as the marines fight their way through hordes of well prepared guardsman. You get a false crescendo as they fight for their lives against the powerful psykers and an apparent release of tension upon their victory. This process starts again with the calm entrance to the inner vault, the meditating inquisitor, it ramps up again over the course of the interaction between the spheres/the molestation of the astartes and then the true crescendo transcends into full blown cosmic horror with the reveal of the entity within.

The whole series ends the rest of the marines squad materializing in the valley by putting sergeant Kohren into visual perspective. This unstoppable astartes is rendered a tiny spec in a vast, incomprehensible alien landscape . its just brilliantly done. In my mind this series is over, there is no need for any more exposition. The ending is left deliberately ambiguous as it leaves the audience in the same state as the space marines - utterly confused and shell shocked. We, like the characters have no understanding of what or where this place is or what brought us here. What are these creatures? Are the marines still alive? Does this place really exist? It captures the nature of cosmic horror so completely and so masterfully. In my mind the visions the Inquisitor was shown are a window into the future, the Astartes die on this "world"