Vigilus Speculative Fiction

What follows is speculation based on watching Chapter Master Valrak and his Primarch obsession in the Vigilus campaign, and me watching a lot of Stan Against Evil recently.

Late stage Vigilus Campaign, the forces of Khorne have descended on the planet driving the Imperial defenders back, things look bleak for the Imperium of Man. Primaris Calgar has requested Roboute assist in the defence as there is talk amongst the front line troops of a huge creature of rage and hatred that none has been able to resist. Word had also been sent to the Grey Knights Chapter, that possibly Angron had returned from the Warp.
Guilliman arrived and organised the resistance against the latest threat to attack the world of Vigilus and the stable gateway to across the Great Rift. The defence was masterful, as always from the Returned Son of The Emperor and soon Angron was exposed, his dæmonic Cohort cut off and stymied by a spearhead from the Grey Knights Chapter and The Sisters of Silence. 
Roboute strode forward to face his 3rd brother to be corrupted by Chaos. Angron towered over his Berzerker followers, seeing Guilliman walking towards him, accompanied by Calgar and his honour Guard, fighting their way towards him, he bellows in rage at the Perfect Son and takes to the air, swooping down on Roboute, scattering Calgar, the Honour Guard and the Berzerkers  they where fighting,  ceramite and adamantium shatter under the cruel hooves of the huge dæmon Primarch as dead bodies are flung aside, brass and red or blue and gold, Angron doesn't care.
"You will die by hand , brother." Angron growls, not a threat, not a boast, just a rage filled, statement of intent. Roboute raises The Emperor’s Sword.
"You are the 3rd of my brothers that I've faced, and the 3rd to make that threat against me, I've defeated Magnus and Mortaion, I will defeat you too." Sword against dæmonic axe and whip, human crafted armour against dæmon fueled rage, even before Angron was raised to the rank of Dæmon Prince this would have been a difficult fight for Roboute, now though it wasn't much of flight. Astartes and Primaris rushed to defend their Primarch and and are struck down with barley a thought by the Dæmon.
" Once again , high rider, I've beaten you." Angron gloats, his cloven hoofed foot, crushing Guilliman into the blood soaked mud of Vigilus." I'll take your skull and add it to my skull Throne." Angron raises his axe to deliver the coup de grace against his brother.
When a sound is heard that was last hears on the home world of The Thousand Sons, it sounds like an ear splitting whistle and everything dæmonic is frozen for a brief second. In that time a freezing cold wind blows from the north and the howl of Wolves is heard on the wind. A giant of a man in storm grey armour , hair that was once a vibrant red, now grey, strides out of the wind, followed by a host of warriors in the same grey armour, disciplined bolter fire driving the warriors in red back, before they stow their bolters and draw their chainsword and charge into the mass of Berzerkers. The giant looks at his brothers and whistles again, Angron and Roboute both look at the returned Leman Russ, as he stands nonchalantly standing there, his spear across his shoulders , arms over the top of it
"Angron, we've been here. Roboute do you require some assistance? " before either being can answer the spare is in his hands, Angron is driven back by the onslaught of blows and Calgar and the surviving Honour Guard form a defensive wall of shields around their Primarch as the apothecaries rush forward to offer their services.