This is an overview of the main bits of the video (I have just about no knowledge on major/or obscure lore so this is just what I am taking away from the video)
It starts with flashbacks/clips of what has happened so far. The next scene is of the two ships (I believe the larger one is the one being boarded). Next is what I can guess is a control room which has a helmet camera of one of the boarding party on the left screen, the right screen shows the inquisitor in front of the machine that is connecting him to the captured orb, the space marine officer puts on his helmet and leaves, followed by the beak helmeted marine. The corridor they walk down to access the inquisitor is coated in purity seals and wards to protect the rest of the ship. The inquisitor is being guarded by a marine and is connected to what looks to be a captured orb with nullifying devices connected to it and is held in place by chains. The next scene shows the door that was reached in the previous video being opened in front of the boarding party, the thing that smashes on the floor is just a bit of decking that was probably broken by the space marines weight. The scene after that shows the front three from behind, on the one on the left you can see a container affixed to his back mounted power plant. Next is the unfinished golden statue which I am guessing was being made as a body for whatever is inside the orb on board the traitor ship, in the background there are floating skulls and spinal columns (most likely servo skulls). Next scene is the orb (I'll call this orb 2 and the captured one orb 1) as the space marines move forward it appears to sense them and activates, it seems to be psychic in nature as there is blue energy moving around on its surface (the previous video represented psychic power with blue lightning/energy), I personally think of it as the mind of a somewhat powerful psychic being trapped inside the orb. As the marines move closer the background noise, in addition to whispering, begins to build up and once the leader moves close enough it seems to stop completely, almost as if they were passing through a force field or just got to a certain range. As they move closer still it says "astartes." and attempts to push them back, it tries this once, barely stopping them for a second, it tries again with slightly more success, then it makes a few weird sounds, each accompanied by a flash of energy across its surface (charging up for a third try), this attempt pushes the marines back a small distance and appears to be achieving something but then it ends having worn itself out. The leader of the squad retrieves devices from the previously noted container and hands them to the rest of the squad, in counter clockwise order these devices are stabbed into the orb (I believe they are psychic nullifiers as they cause the psychic energy to disappear in a slowly spreading zone around them) It looks like the marines are trying to capture orb 2.
The inquisitor is pulled in to the conversation between orb 1 and orb 2 this is what is what I think is said
Orb 1: "Who's here?"
Orb 2:"I have failed brother"
Orb 1:"We have all failed"
Orb 2:"The astartes deny our touch. You must return. Break your shield"
Orb 1:"Impossible, We'll never survive. You must take them"
Inquisitor turns to marine officer "Return them immediately!", officer turns to guard who runs off to get a message to boarding party, the inquisitor slightly jumps, as if struck (most likely psychic attack from orb 1) and his hands and body begin to shake. Orb 1 charges up energy and forces itself into the inquisitors mind causing sparks to fly off the cables and the inquisitor to stand up. Marine officer walks forward, inquisitor turns to him as the cables attached to his visor disconnect, a scene is briefly shown that appears to contain a gathering of people in similar uniform to the psykers from the previous video starring at a line of energy or maybe a star (possibly something ctan related except I know essentially nothing about ctan/necron/necrontyr lore so I can't say for sure). The next scene shown is of four marines/figures standing in the palm of a massive statue with a skull for a face, the background contains cliffs with lava or molten metal or some other liquid falling, there is another giant skull further back. The last of the rapidly shown scenes is of a marine having his flesh ripped from the bone by some unseen force. The inquisitor begins screaming as light blue light emanates from his mouth, nose and eyes, and blue lightning (presumably psychic power) courses across his head, I'm guessing that this represents him beginning to be possessed or that he is about to explode as a psychic bomb. The marine officer grabs the inquisitor by the head, cracking his visor, and punches him in the face, entirely pulping his head, the beak marine then shoots the body four times. The video cuts to the marines standing around orb 2, the leader looks towards the viewer and muffled speech can be heard, presumably the warning being received by him over the Vox but before he can order a withdrawal orb 2 suddenly glows bright green, he looks directly at orb 2 and in a flash of light the background goes black and the orb is replaced with some cool looking creature that I am in no position to speculate about, the creature rapidly moves back into the darkness, followed by some tentacles that appear to be its limbs. Orb 2 pulsates slightly, the leader of the squad puts out his arm to steady himself as the orb pulls in all the marines, the rest of the squad were unprepared and all get caught against orb 2. As the squad leader realizes that the orb is actually pulling his hand into its surface, he reaches for his plasma pistol, but because he is no longer supporting his body with two arms he is pulled further in, as the barrel of his plasma pistol is pulled in he pulls the trigger, the plasma shot explodes back out of the orb, incinerating his hand and he is pulled inside. Next scene is a first person shot in a murky darkness from the squad leader as he looks at his left arm, and then the stump of his right arm, as he looks back at his left arm a tentacle appears, wrapped around his forearm, this tentacle starts glowing light blue, this glow rapidly moves away from the marine, along the tentacle, back to its source, the background lights up as a cloudy sort of orangish red and the tentacles can be seen leading back to a slightly humanoid creature with a bright blue glow its center, the other four space marines can be seen floating and connected to the creature via other tentacles, there is also a lump of something floating near the bottom right of the screen which might be the remains of the plasma pistol and the squad leaders hand or its something else entirely. A dark cloud or form emerges from the bottom of the screen and engulfs the creature with the tentacles (killing it or disrupting it) the creatures tentacles were not covered by the dark shape and with a screech/scream then slither away. The last scene shows the squad leader teleport into a desert landscape, fall to his hands and knees, take off his helmet and cough up either blood or vomit, as he stands the camera shows that he is in a gargantuan ravine/valley with sides composed of giant skeletons, at the base of the sides there are giant skeletons wearing ornate clothes and there are some pillars further in from the edges of the cliffs there are some similar flashes to the one that caused the squad leader to appear on three of these pillars and a figure can be seen appearing on each pillar for each flash. Just guessing here but I think that in one of the flash scenes shown just before the inquisitors death (the one with the four figures standing on a massive hand) that it was of the future and it was showing the marines exploring the location they got sent to, the other scene with the dead marine might be showing what happens to the marines, the first of the flash scenes could be the psykers/leaders of the rebellion trying to make a deal with whatever is inside the orbs. One last thing, I'm rather sure that the unfinished statue, the orbs, and the psykers masks are all mad of the same material (possibly a psychically sensitive material).