The Sons of Horus were the arch-traitor legion, though Lorgar's Word Bearers planted the seeds, but they had one of the best records of them all in the Great Crusade. Cthonian ferocity was harnessed into the legion, supplementing the gene-seed-implanted ferocity of the original Terrans that made such an impact on  the Luna campaigns. 

Under Horus, they were throat-slitting killers whose instincts were put into the service of Humanity. But they also venerated their Primarch, shouting "LUPERCAL!" as they sprung into battle. Some of that was on display as the small group of Luna Wolves in this ragged band handled the perimeter duty and silenced sentries brutally with fist and knife, which was a return to the hive gang war tactics some of the recruits, had they said anything of their origins, might have learned in their upbringing. But now, they faced a different situation and a tactical shift was in order as the noise came from the Chateau and the bolters were unlimbered as they loped toward their final target, approaching the Rhinos that the Dark Angel guarded. 

Prodigal Son may well have been a silent leader, at least to the outsiders, but he was a tactician to the bone, and he'd positioned himself to come to the aid of other Space Marines when the unexpected happened. They would never say the name of their Primarch at all, but they still knew how to fight.

A bolter shell was a mass-reactive shell that left a horrific and gory wound that was terminal, particularly to a normal human being like the militiamen. And yet, the fire did not bring the enemies down; his own bolt barked out of the barrel and found its mark, except the mark was growing exponentially, expanding like a balloon, with red-glowing eyes and eldritch energy encircled them at wrists and ankles, hauling them into the air. There was a scream of ethereal rage as these human beings twisted beyond comprehension and sanity, to the horror of the Astartes near the rhinos, veterans of the Crusade and even the opening blows of the Heresy, were unfamiliar with.  

They had seen mutations, but this was different and smelled of sorcery; the men burst into a spray of blood and from the spray of blood, suspended in the air, came a portal. And from that portal came, howling, sword-bearing horrors that rushed at them with a terrible ferocity and speed. The flamer carried by one of their number unleashed in a long burst, trying to catch them as the bolters were fired rapidly. A piece of the Prodigal Son's mind gibbered, but the other part bellowed "AND YOU WILL KNOW NO FEAR." 

But if the bolter fire shredded the first ranks, multiple hits ensuring that they stayed down, there were enough left to engage the rest of Prodigal Son's small detachment. The Luna Wolves were fluid and instinctive in their shift of weaponry, from the ranged combat role into preparation to defend in close combat, and ferocious in meeting the charge. It was chainswords against those demonic blades, even while the whispers rang in their ears, calling them to their rage...