Ok to sum up the ending, the Iron Warriors are looking for a chaos artefact because Perturabo is peeved off with people betraying and spying on him and he wants to make his legion strong again in this new galaxy, he knows the Alpha Legion is about and after the same thing as him, he's been sending patrols out to look for the artefact but they go missing which is eventually explained as the Alpha Legion destroying them in some shape or form. the Vanus assassin has been hunting the Alpha Legion operative Jalen, turns out there's three of them which are triplets, one gets executed by Ardonis after the Vanus fakes a distress call by Ardonis, as Ardonis was captured after finding out about what they are searching for, Ardonis gets annoyed at the Alpha Legion being secretive and puts two bolts in the eyes and one in the mouth of Jalen (just before that Jalen figured out he'd been tricked by the Assassin).
Ardonis subsequently kills several Iron Warriors and escapes the base just as the Iron Warriors and Loyalists are about to undertake one final massive battle. The second Jalen ambushes the Vanus as she watches Ardonis kill the first Jalen, it's then when she reveals she knew about the triplets, she'd sent a warning and the Iron Warriors show up and take out the Alpha Legion including the second Jalen. She then jumps to her death to avoid being captured by the Iron Warriors but it's a long way down.
After taking out a moderately sized company of loyalist armour including a baneblade and stormlord (who wrecks all the predators and a dreadnought), Hrend comes across some Alpha Legion tanks, Perturabo warned him about spies and after faking comradeship and heading to their target location, shoots them in the back and wipes them out. The Alpha Legion's messages were blocked by the Vanus assassin and altered to make it sound like they'd lost the Iron Warriors.
Khord after having made it mistakenly to the major command centre of the Loyalists gets arrested as a suspected spy but then gets let out by an Iron Hand who commands a Predator, they head off after the Iron Warriors patrol. Hrend and his men find the place of the object and begin to dig, executing the astropath who guided them there. The drill cracks and Hrend decends he comes across a black wall that could be part of a round structure, it has pictures of Daemonettes on it and a fake Perturabo tempts Hrend with visions of what the Iron Warriors could become if they give in and accept chaos, the images are of them looking like molten Iron killing all in their path. Hrend refuses and subsequently explodes, I can't be sure whether he trigged his own self destruct or whether chaos killed him I'll have to read the passage again. Prior to the explosion Khord in his Malcador and the Iron Hands tank shows up and begins a battle with the Iron Warriors. Kord's tank gets flipped on it's side and is unable to move, then the explosion erupts and everything but the Malcador tank seems of been destroyed. The scout who'd been with Kord from the beginning (called Origo) tells him to call out call out and they will hear him, his brothers are dead but they will hear him (this leads me to believe Origo is the third Jalen brother though I'm not sure.) He calls out but the Vanus is listening (she had been manipulating Khord the entire time) and she's close to death, she either has the choice to let the signal broadcast as she had intercepted it, or she cuts it. She decides to cut it.
That's the end of that path, the other part is Argonis makes it to the Iron Blood and confronts Perturabo and Perturabo finally comes clean with him saying he was doing it for the sake of the Warmaster, but Argonis is having none of it, and finally unlocks the Astropath that has been following him about, turns out to be one of those Davinite conduit guys and Horus looks on and explains he's been watching his Emissary the whole time and begins to berate Perturabo demanding he recall his troops and leave Tallarn, then he tells Perturabo to kneel before him.
The final paragraph describes how the Iron Warriors packed their bags and left.