Astartes V: Explained!
Many people had questions about what we just saw. The Retributor chapter of space marines we have been following finally reached the vault. Inside the vault, they found an artefact. It can be easily identified based upon their material composition and their iconography. Almost all such orbs are constructed of a crystalline/ceramic material that resembles natural or fossilized bone, with twisted and asymmetric symbols that distort and shift. This is an Artefact of Doom of The Yu'Vath empire.
Who? The Yu'Vath were an ancient and corrupted alien empire that inhabited the Calyx Expanse two Terran millenia ago, during the Angevin Crusade of the early 39th Millenium. These evil xenos were twisted servants of the Warp, who built their empire and technology with dark sorcery, soul-slavery and daemon worship. Thought to be purged completely in imperial records, whispers in the Koronus Expanse, claim however, that they were never truly defeated, and Yu'Vath Hell-worlds still exist hidden within the darkness of the Calixis Sector.
What? "There is a corrupt logic present within these artefacts, but to understand it would be to risk corruption of the soul and the mind. The Machine God has no tolerance for such blasphemies" - Magos Mortigen Hale, Disciple of Thule
Psychic Sphere Leftover weapons and devices of The Yu'Vath are knowns as Artefacts of Doom. This particular device is a techno-warp construct, constructed of dark energy. As much of The Yu'Vath's technology, it is sentient and more sorcery than machine. These orbs may cause visions and dreams of a great dark presence. If living creatures are effected by these visions or dreams, they become possessed. Ordinary citizens or psykers alike will begin attacking their family, friends, neighbours and striking at the very infrastructure of the governmental, or planetary systems themselves, like an organized army. Possessed individuals become extensions of the alien construct which can draw on their past skills and memories, but extensions nevertheless. If a Psychic Sphere is destroyed, those turned by the dark visions will recover - although they may never be trusted by the Imperium of Man again, and executed. Psychic Spheres are known to be unnerving to talk to, or even to be near when their true nature is known. They can counterfeit emotions, and even show fear, to accomplish their goals.
What happened? The Retributor chapter of Spacemarines have just killed some soldiers and psykers protecting a vault. Inside they come across an Artefact of Doom locked in a vault. The Imperium of Man has long since forgotten the terrors of The Yu'Vath, and the Inquisition knows only weapons of The Yu'Vath are scattered around the sector. Meanwhile in space, on a Battle Barge of the Retributor Chapter, Veterans of the Retributor Chapter of Spacemarines(Cloak/beaker helmet) oversee a Sanctioned Psyker(Good guy sorcerer) of the Inquisition tap into a Psychic Sphere thats ALREADY ON THEIR SHIP! The Veterans see their battlebrothers on the monitor get into the vault and finding another Psychic Sphere. Meanwhile, the Battle Brothers of the Retributor chapter walks past a giant golden statue/robot, prone on a platform. The Yu'Vath is known for being able to possess inanimate objects with Warp-Sorcery. Sacreficed souls fused to golems and the like. The battle brothers finally approach the second Psychic Sphere. The Sphere says "Astartes". Last time the Sphere saw them, Astartes nearly wiped their empire out! The Psychic Sphere releases a psychic blast trying to mind control them. The brothers move forward. A second more powerful blast tries to mind control them. The battle brothers struggle for a moment, but move forward. Lastly, the Sphere tries to blast them away and shield itself. They move forward and reach it. The Retributors pull out Transmitter Relays to connect the Spheres together. The Inquisition is probably trying out the weapons capacity of these "Artefacts of Doom". When the two Spheres are finally connected, the Sanctioned Psyker of the Inquisition connects mentally to both Psychic Spheres, trying to control them. The Inquisitor finally loses the battle, he sees the vision. This is the process in which Psychic Spheres take over individuals. The spheres now talk together. Who is here?, We have all failed. They have failed to build an army to rekindle their empire. The astartes deny our touch. The Retributors are too strong to mind control! You must return. The Sphere on the ship wants the one in the vault to escape to a hell-world before he to is captured. Break your seal. The Psychic Sphere on the ship finally has a mental path out of captivity. We'll merge our minds The Spheres merge minds so the one on the ship can finally escape. You must take the alpha. The Inquisitor loses the battle and is controlled by the Psychic Sphere. Right before our brave Sanctioned Psyker loses the battle, he realizes what these artefacts are, are screams "Bring them back". The veteran immediately calls to bring the Retributors back from the vault, but it is too late. The Inquisition Psyker is possessed. The Veterans immediately execute him(Possession is common in this universe, and marines will never risk more than a second of anything that looks like it. This was a mercy kill trust me.) The Psychic Sphere in the vault is now the merged personality of the Sphere on the ship and in the vault. The tentacle presence in the sphere now starts to warp travel away. The marines get sucked into the orb, as the inside of the orb is now a Warp rift to a Hell-World. All retributors are killed(The sanctioned psyker has a vision of the spheres' plan where he can see their death in the warp), except the "Alpha", our protagonist. They have a plan for our main Retributor marine(The one with the red eye broken visor). Finally, the Spheres teleport back to a Yu'Vath hell-world, where countless kings of the Yu'Vath sit on their corpse thrones, ready to return their empire to glory by being literal ghost entities.
Tl:DR; Retributor marines encounter Psychic Spheres of The Yu'Vath. Psyker gets mind controlled and executed. The Sphere teleports to a Yu'Vath Hell-world killing all spacemarines but our protagonist.
Edit: Grammer and mispelling