The Sub-Sector of Ultramar domain of the Ultramarines Space Marine chapter. Ten thousand years ago, it was known as the Five Hundred Worlds, conquered and ruled by the Ultramarines Primarch, Roboute Guilliman. When the Emperor rediscovered Guilliman during the Great Crusade, Ultramar joined the growing Imperium of Man as an empire within an empire. After the Horus Heresy, Guilliman sundered the Space Marine Legions into chapters, including his own. When Guilliman fell at the hands of his daemon brother, Fulgrim, and was placed on a stasis throne that lay in the Temple of Correction on the Ultramarine homeworld of Macragge, it marked the end of an era for both the Imperium of Man and Ultramar. Sometime before his fall, Guilliman disbanded the Five Hundred Worlds and allowed hundreds of worlds in the Realm of Ultramar to declare their autonomy until only a small core of 11 planets remained officially part of what had become an Imperial sub-sector. Only a few months after the Fall of Cadia, Ultramar was under siege by the Black Legion and their many allies in the 13th Black Crusade.
The Black Legion was formally known as the Luna Wolves, and then the Sons of Horus were first the Legion of Warmaster Horus Lupercal. After his annihilation at the hands of the Emperor of Mankind and the lose of the war. The Sons of Horus fled into the Eye of Terror, where they would be remade into the Black Legion under the banner of a new master, the First Captain of the Legion and new Warmaster of Chaos Ezekyle Abaddon, now known as Abaddon the Despoiler. The Legion is now an amalgamation of multiple traitor Space Marines from multiple legions and, in some cases, even xenos, though this is rare. Daemons are far more likely. Abaddon's Black Crusades have been the bane of the Imperium for ten thousand years, with the previous twelve always being hailed as imperial victories. In reality, they have been preludes that allowed Adaddon to complete goals and gather the resources he needed to execute his masterstroke, the 13th. Abaddon calls his great plan the Crimson Path. The Lord of the Black Legion was informed by the Chaos Sorcerer Zaraphiston of a disturbing possibility: the possible return of Primarch of the Ultramarines. Not only that, but the tides of the Warp have been whipped into a frenzy by the coming of a powerful new foe that the gods will not speak of clearly to their mortal servants, but one name echoes through the Immaterium: The First. Abaddon cannot be sure about how Guilliman could be brought back to life, but he knows that one certain way to stop it is to destroy him, Macragge, and all of Ultramar. With the Great Rift in place, what is left of the Ultramarines, the Macragge Planetary Defense Force, and a few allies near enough to come to their aid are all that is left to defend their stricken Primarch. He dispatched a portion of his Black Fleet to get the job done.
While a massive space battle rages in orbit over Macragge, Chapter Master Marneus Calgar coordinates the defense on the surface even as he fights. Chapter Master Calgar is one of the longest-standing and most respected Chapter Masters in history, a true Hero of the Imperium. The chapter fortress-monastery, the Fortress of Hera, is under siege by hordes of Black Legion, their human and abhuman slaves, and their daemon allies. The fortress is the main focus of the attack, but the enemy is attacking across the entire planet in an attempt to spread the defenders as thin as possible. They have found a much stronger defense than they were expecting. Half the strength of the Ultramarines was able to return to Ultramar before the Great Rift made the Warp nearly impossible to navigate. Several elements of the Imperial Navy also sailed into the sub-sector looking for a safe harbor, bringing several regiments of Astra Militarum. The same need for safe harbor brought several Space Marine Strike Cruisers from other chapters. Their calls for aid, though deadly to their astropaths and largely useless, managed to bring in the aid of their successor chapters, the Novamarines and Iron Hounds. In a surprising turn of events, they also find themselves fighting beside Saint Celestine, who was brought to Ultramar aboard one of the Ultramarine's own Strike Cruisers, fleeing the destruction of Cadia. Her presence brings the knowledge that the Emperor is with them, which spurs the courage of the mortal troops and many of the Space Marines.
Marneus wades into a squad of attacking Black Legion, firing the built-in bolters of the Gauntlets of Ultramar with his Honour Guard at his side. He is clad in his master crafter Terminator Armour, the Armour of Antilochus. He blasts two of them to pieces before smashing another three to bloody ruins with his power fists. His Honour Guard, also clad in Terminator Armour, rips through the rest. They are fighting their way toward the Temple of Correction. The Ultramarine Third Company stationed there is reporting a major enemy push. The thought of them breaking through and reaching the Shrine of the Primarch has Marneus hailing ass and obliterating everything in his path. A figure in golden armor swoops down on white wings and cleaves the head from a Black Legion captain near the back of of a fresh rank of Black Legion charging Marneus. Celestine's attack throws the other traitors into disarray, and Marneus doesn't give them a chance to regroup. He and his men crush them and continue their sprint. Saint Celestine flies above them on her psychically created wings.
"Chapter Master," says Celestine over the vox, "I heard about the Temple of Correction. I am here to offer my aid."
"It is welcomed, Lady Celestine," replies Marneus as he fires a barrage into a hoard of beastmen, killing over a dozen before he crashes into their ranks, "This scum is nothing but cannon fodder, but wading through their blood and guts is like wading through mud."
"Let me help clear the way."
Celestine soars high into the air before streaking down like a missile and crashing down into the center of the beastman ranks, releasing an explosion of psychic force in a forward wave that blasts the beastmen back like a tidal wave. Behind her, the Ultramarines break through the ranks before them. Marneus stops and offers his hand to Celestine. She takes it and stands up.
"You truly wield the might of the Emperor, my Lady Saint," says Marneus, impressed.
"Only a fraction of his power, Chapter Master," replies Celestine as she catches her breath, "I will expend every bit of it in aid of his great Angels."
"You have the eternal gratitude of the Ultramarines," says Marneus, "Can you continue?"
"Yes, I-"
Her words are cut off as a wave of Warp energy washes over her. Even the nonpsyker Ultramarines can feel it. All around them, the corpses of the Black Legion and beastmen shift, melt, and start to flow together like sludge. They come together before the group of warriors, creating a towering armored corpse centipede with bladed limbs made of bone and ceramite and a crocodilian maw with an endless amount of black dagger-like fangs. The daemon centipede lets out a laugh that sounds like the death rattle of a thousand babies before it speaks in a voice that rumbles like an earthquake.
"Kagath lives!"
Behind the daemon comes more Black Legion. The power of the Warp twists these. The Ultramarine and Celestine raise their weapons and prepare for a much harder battle.
"God-Emperor, give us the strength," says Celestine as she calls on her powers, making her blade blaze, "To wipe these unclean abominations from the face of your Holy Imperium."
Kagath laughs again, "Oh, you are filled with his dimming light, aren't you, little bug? Good. Good. There is nothing better than crushing the hope of a true believer. Especially the faux saints of the Corpse Emperor. It is like eating a star that can scream."
The daemon charges forward on its dozens of legs like a train as the Black legionnaires open fire. Marneus grabs Celestine and dives out of the way of the daemon's charge while absorbing the explosive round on his Terminator plate. A member of his Honour Guard snapped up by the daemon's maw. The rest fan out and return fire on the Black Legion and the daemon. Marneus releases Celestine, who takes off into the air, while he opens fire on Kagath, who is turning to charge again. The heavy bolt rounds explode against the daemon's armor plate, doing no damage. The top half, Honour Guard Titius Invicterius, flies from the daemon's maw and strikes one of his brothers, knocking him down. Marneus watches as Celestine drops onto the thing's back, plunging her blade into it. Kagath screams in pain before performing a death roll that would have crushed Celestine if she did not launch herself off in time but she is forced to leave the Ardent Blade, buried in the daemon's back. Kagath gets back to its feet and leaps into the air, snapping after Celestine, who avoids the by an inch. The daemon's landing shakes the ground and cracks the white stone. It lets out a roar and charges again but then almost comically slides to a stop, burying its bladed legs into the ston to do so.
Standing before it, having appeared seemingly out of nowhere, a white-haired man in black and gold power armor with a winged half mask over his face. The man is dwarfed by the massive abomination but stands before it without fear. Marneus stares in shock as the daemon slowly begins to step back from the man. The white haired man reaches over his shoulder and pulls a blade so black that it seems to eat the light around it. He spins it fluidly, and the air seems to shimmer as the blade cuts through like it refuses to touch the blade.
"No," says Kagath as it continues to back away, "No. No. No. You cannot be here. You are not real. You can't be real. The Anathema… the Anathema, has no father. You are not real!"
"Interesting," says the white-haired man in a voice that sounds like sunshine, darkness, and fate, "I feel real. I am a father to many, including him. I am here. I am Him. You, on the other hand. You are gone."
The white-haired man swings the sword, and there is a crack like a whip moving at the speed of sound as it slices through the air. Marneus can clearly see that it doesn't touch the daemon, but then there is a blast of thunder, and Kagath is split in half lengthwise. The daemon burst into glittering white ash before disappearing completely as if it were never there. The white-haired man spins the blade again before returning it to his back. Then he pulls the two black and gold pistols on his hips. He doesn't move, but he disappears. When Marneus looks back to where the Black Legionnaires are standing in silence, too shocked to fire, the man is there before them. He raises the guns and fires so fast that Marneus can barely follow. The guns fire green energy that hits the traitors and blows them into the same sparkling white ash that disappears in seconds. In less than three seconds, twenty Traitor Space Marines are just gone. The man spins and holsters his guns before turning around and walking back over to Marneus. He stops before the chapter Master looking up at him with eyes of pure gold.
"So, this is Macragge," says the white-haired man as conversationally as if he didn't just kill a daemon and twenty Space Marines in the span of ten heartbeats, "Beautiful place, but you seem to have quite the infestation."