The Horus Heresy novels have taken great pains to tell us that the Heresy was the greatest tragedy of Warhammer 40,000. They also weren't lying when they said that it was "a time of legend".
Book 1: Horus Rising
The opening book, Dan Abnett's Horus Rising has Garviel Loken rush in and avenge his senior commander Sejanus. After killing tons of invisible baddies, he meets the "false Emperor" who proclaims that the Imperium is wrong in its doings. But Loken rebuts it perfectly by pointing out that the Imperium had before tried reasoning with the planet.
Loken himself is just full of awesome. One memorable moment: After one of his commanders (Jubal) is possessed and kills another marine (which is a cardinal sin, by the way) what can Loken do? He can't shoot Jubal that would be breaking the oath of the Imperium. So he does the next best thing he wrestles the gun out of Jubal's hands and nearly gets thrown off a cliff. Right after he gets up Jubal draws his knife. And thus it breaks into a knife fight. After even beating Jubal, Loken tries to keep him alive. Before he is forced to kill him.
Whilst in a sparring match with a Lucius (the proud sword-fighting member of the Emperor's Children). Loken beats him by grabbing his sword hand and punching him to the floor.
Which is not to say that Lucius doesn't have his moments. Lucius challenges Loken to a rematch in the next book, and wins. Also, during heavy combat with the Megarachnids, Lucius picks up a limb of one of the fallen, and uses it as a weapon.
Iacton Qurze yes he was treated like a doddering old fool of a space marine but he was actually a lot smarter than he looked and on his own managed to break the back of a megarachnid invasion wave.
Saul Tarvitz (who eventually befriends Loken) survives an attack by bladed-spiders and vicious birds.
A particularly interesting moment. Tarvitz accidentally destroys the source of the transmission jammer when he uses explosives on trees because he didn't like the way the bodies of soldiers were hanging on them.
Nero Vipus slices off his arm after it was injured saying that he "threw it away"
Hell, the opening of Horus Rising is one. The description of the Luna Wolves attack (a front of charging Astartes three kilometers wide) just lets you know how epic the series' battles are going to be.
The moment in Horus Rising where Euphrati Keeler first utters a line that every Warhammer 40K fan should know.
Keeler: The Emperor protects!
Book 2: False Gods
Book 3: Galaxy in Flames
So, how many characters do you know who have set a whole planet on fire? Because I only know of one: Horus mother***ing Lupercal.
The last stand of the loyalist World Eaters on Isstvan III. After learning that they've been betrayed by their own Primarch, said Primarch, Angron, comes down to finish the job face to face. Angron and hundreds of Traitor World Eaters are charging straight for the Loyalist World Eaters, who are facing certain death, so what do the loyalists do? They counter-charge their own Primarch!
The speech Loken gives to the loyalist members of the XVI Legion when Horus's treachery is revealed, culminating with Loken declaring that the loyalists are no longer the Sons of Horus, but the Luna Wolves.
The entire battle of Isstvan III is one huge Dying Moment of Awesome for Saul Tarvitz. Managing to discover the traitors' plan and reach the world's surface with help from other loyalists, he turns what should have been simple annihilation of the loyal elements into a full scale battle. Even after being betrayed by Lucius and an Imperator class titan deployed at their position, he manages to lead them into final battle which massacres over a third of the traitor legion's numbers.
Book 4: The Flight of the Eisenstein
The last words of Huron-Fal, the Death Guard dreadnought in Flight of the Eisenstein. He's been betrayed by his Primarch and his Legion, the city he's in is being hit with a virus so potent that it liquifies those it infects, and he's been infected, so what does he do? What any good Space Marine would do, of course! overload his reactor, saying "This death... this death is ours. We choose it. We deny you your victory".
Can't forget Nathaniel Garro if you mention this book. While his one-on-one fight against a Great Unclean One near the end is awesome on its own, what turns the tide of the Heresy is to, after being thought as a heretic who defames Horus, and knowing that he could be killed for it, pull a What the Hell, Hero? on Rogal Dorn, who almost kills him after being questioned. And he does so anyway, because he knows he is right.
Nathaniel Garro: (To Rogal Dorn) Are you blind? [...] I asked if you were blind, lord, because I fear you must be. Only one struck by such a terrible ailment could be as you are. Yours is the blindness that only a brother might have: that of a keen judgement clouded by admiration and respect, clouded by your love for your kinsman, the Warmaster.
His defiant words to Malcador the Sigillite is equally awesome, demanding the Sigillite to give them a role to fight against Horus. The speech pretty much explains his character
Nathaniel Garro: We cannot stay here, watching the stars and waiting for the day that Horns comes seeking battle. I request...No, I demand that we be given a purpose! I am an Astartes, but now I am a brother without a Legion. Alone, I stand unbroken amid all the oaths that lie shattered around me. I am the Emperor's will, but I am nothing if He will not task me!
Malcador the Sigillite revealing that he's recruiting Garro and his fellow warriors for the precursor organisation that will eventually become the Inquisition.
Book 5: Fulgrim
Book 6: Descent of the Angels
Book 7: Legion
In Legion a soldier named Dinas Chayne, head of a special bodyguard squad, manages to STAB Alpharius with an unpowered sabre, through his power armour, and causes him to bleed a little. Of course Alpharius simply cuts him in half lengthwise after that, complete with a pithy one-liner.
Can we just clear this up: a Badass Normal fences a post-human demigod to a standstill, but then gets his sword stuck in his opponent's Nigh Invulnerable power armour whilst injuring him - an opponent who can shrug off blows that would liquify Chayne. Fucking badass. Unfortunately, futile.
The sheer Magnificent Bastardry of Alpharius' meeting with the Cabal. First, he recieves the undying loyalty of Grammaticus' best friend and his lover. Next, he equips them both with teleporter beacons so that, when Grammaticus "escapes" to warn the Cabal of their millitary incursion, he leads the Primarch and his best troops straight to them.
Three words: "For the Emperor."
At the climactic naval battle, an entire Imperial Fleet is desperately fighting a single Alpha Legion battle barge. After losing dozens of vessels to this single threat, the commanders try to escape. Cue another Alpha Legion ship.
Battle for the Abyss Fifty loyalist Space Marines aboard a small fleet of ships which is quickly destroyed vs the most powerful Mechanicus created warship ever devised, containing alpha level psychers and tens of thousands of Word Bearers. You better believe that some serious COMAs happen.
Book 9: Mechanicum
In Graham McNeill's Mechanicum, the Emperor himself gets one, although it's told second hand. In the 11th century, the Emperor manages to defeat and BIND the Void Dragon. It ought to be mentioned that the Void Dragon is the most powerful of the C'Tan star gods (just to put it in perspective, a weaker C'Tan, the Nightbringer, was capable of defeating the collective Eldar race and their actual Gods in combat). All the more reason why the Emperor is a walk CMOA.
It gets better. While the Emperor's paranoia and arrogance hand him the Idiot Ball on various occasions afterwards, here he's in perfect Magnificent Bastard form. He knows, by prescience, that many technically-oriented people will come to Mars in the millennia to come. So he binds the Void Dragon there, in a place few will go, just constricted enough that its technology-focused influence will inspire great works of engineering and seed the idea of a Machine-God without enough contact to make the resulting Machine Cult actually loyal to the Void Dragon.
Another thing to remember is that fans will try to take away some of the awesome of the victory by pointing out that the Emperor, belovedby all, only defeated a shard of the Void Dragon after it had been hit with several shots from a few Blackstone Fortresses, and that the only way to normally do this is to throw so much firepower at the C'Tan that its necrodermis shell breaks apart, causing a massive explosion that kills whoever destroys the C'Tan. Those are retcons which came after the book was written. As far as Warhammer 40,000 canon was concerned, when Mechanicum was written, the Emperor of Mankind fully defeated a C'Tan star god by himself without destroying half the planet.
Book 10: Tales of Heresy (Anthology Series)
After De'shea is in its entirety a CMOA for Kharn. And it's not his martial prowess he earns it with, but his brain. After Angron, freshly brought to his flagship from a Last Stand that killed all of his old comrades, in the grip of guilt and rage, killed the first seven Captains of the War Hounds, Kharn (the 8th Captain) manages to get through to him and convince him to accept the War Hounds as his legion and to reverse his opinion of the Emperor. While being flung around like a ragdoll and being beaten within an inch of his life and never fighting back. All the more impressive because while they talk the same language, Angron grew up on a primitive world as a gladiator slave - and as such doesn't understand most of the concepts Kharn takes for granted. And Kharn manages to bridge the cultural difference and translate. While every bone in his body is being shattered.
The Priest in The Last Church seeing through the Emperor's talk revealing him for what he is. He knows the Emperor better in a space of an hour than others do in years.
Book 11: Fallen Angels
Book 12: A Thousand Sons
Book 13: Nemesis
Book 14: The First Heretic
The First Heretic gives us our first view of Corax in combat. Beating down a dozen daemonically possessed Word Bearers he proceeds to almost kill fellow Primarch Lorgar, all the while demanding he explain his treachery and is only stopped by the intervention of the Night Haunter.
To complement that moment we have Lorgar ignoring the words of his tutors and rushing off to save his Possessed Space Marine sons form being slaughtered by Corax and unleashing his full psychic potential. Doubles as a Heartwarming Moment.
Incarnadine'sBig Damn Hero moment near the end. Sure, it failed. Sure, it was on the side of Chaos. But facing down four Custodes, giving them a fight for their lives, and killing one of them is one hell of a feat.
"I always hated you, Xaphaen"
Let's give some context for this one. After all the other Custodes had been killed by the Possessed World Bearers, Sythran faces down the remaining six and, before accepting his inevitable demise, hurles his spear through Xaphen, who had been a Jerkass throughout the book. Then he takes off his helmet and breaks his vow of silence to deliver a Bond One-Liner before the others kill him. AWESOME.
Book 15: Prospero Burns
While in later novels, the Vlka Fenryka tend to suffer from the Worf Effect, in Prospero Burns, they get plenty of moments to shine.
The first time we meet a Space Wolf is when 'Bear' comes in. He quite effortlessly dispatches a hundred, ferocious Fenrisian hersir. That isn't the awesome. The awesome part is when he kills a gigantic sea-monster, a ''hrossvalur', like he's chopping wood.
Later on, we are treated to an example of the Vlka Fenryka displaying their ability to simply not be noticed. They drop through girders and gangways of a small, metallic moon, to drop down onto the heads of essentially a bunch of super-cyborgs. Without making a sound. Fifty superhuman wolf-men moving against metal girders, flowing down like a liquid, and it is specifically mentioned that they are utterly silent.
Of course, then we see why exactly, even on a strategic scale, the Wolves of Fenris are basically the most ruthless legion. Seeing the immense problems the Imperial Army is having taking the planet, the Jarl of Tra takes that same huge orbiting planetoid, about the size of a small moon, and smashes it into the enemy planet. He essentially caused a minor apocalypse simply because it was the most effective way of ending the conflict.
Every Legion, however, has to have it's grand finale. Prospero Burns gets the Burning of Prospero, and it highlights both the absolute tragedy of the effect, and the reality of how fucking awesome the Space Wolves AND the Thousand Sons are. The only reason they do not utterly wreck each other is because the Thousand Sons deliberately hold back on their orbital defences and magical capabilities, and so when the Space Wolves begin tearing apart the Thousand Sons, it is implied that they fight brilliantly and magnificently, as only Space Marines can, but simply aren't a match for the Wolves. After all, this is what they were made for.
The end of Prospero Burns when The Chaos Demon reveals itself, cutting its way through the Space Wolves, only to be stopped by Bear. Bjorn is one of the greatest heroes of the Imperium for a reason
Book 16: Age of Darkness
Iron Within is CMOA for Warsmith Dantioch.
Let's mention how he and his 30 loyalist marines, one Dreadnaught, a platoon of Imperial Armymen and some gene-spliced miners managed to hold off the might of the Iron Warriors, multiple traitor Army forces, and at least one Legion of Titans ("The God-Machines") for over a year. This was achieved by building a base on a hostile planet... down a hole that could only be accessed by dropships for twenty minutes at a time, from a cavern BELOW the base, which could at will be FLOODED with burning promethium, constructing the base so that each segment was autonomous and could hold out indefinitely, then, when the enemy forces were at their most committed, detach the base from the ceiling of the cavern, plunging it into a burning lake of oil. Then escaping and stealing his enemy's flagship. Oh, did we mention he did this whilst artificially aged to the point where he should have been dead?
The escape of the Raven Guard from the Isstvan system in Deliverance Lost. Spending hours dodging continual weapons fire from a small fleet of enemy ships, with nothing but their stealth systems to protect them, Corax decides that rather than just flee he is going to get some measure of revenge. He slowly positions his battle barge over the Word Bearers flag vessel of the pursuing force, decloak and jump away. This drags the enemy ship unshielded into the warp, where its crew last just long enough to realise what has happened to them.
Book 19: Know No Fear
Know No Fear, AKA "Dan Abnett's argument on why the Ultramarines are still Badasses", is absolutely crammed with this:
Roboute Guilliman vs Kor Phaeron: While Kor Phaeron is waxing over how he will use his athame to corrupt the Primarch of the Ultramarines to the service of the Chaos Gods, an injured Guilliman says that Phaeron made the mistake of not killing him. Roboute Guilliman then plunges his Power Fist (which has lost power at this point) into Kor Phaeron's chest, then shows the Master of the Faith his own black heart. An impressive moment for someone who has been suffering the fandom's hatred for several years now.
Let's give Kor Phaeron some credit here too. While Guilliman got the last laugh by being far more loyal than Kor Phaeron could have fathomed, there is no getting around the fact that Kor Phaeron didn't just fight, he beat up a Primarch, one of the demigod-like sons of the Emperor, and had him at his mercy. To date, he is the only mortal being (Aka not a Primarch, not ascended to Daemonic levels, or a Daemon itself - hell, not even a full Astartes) to achieve such an incredible feat.
Guilliman elevates himself from The Scrappy to the rank of Crazy Awesome by punching a Word Bearer's head off. On the hull of his flagship. In space. Without a helmet.
The horrific but amazingly demonstrated effect of what an accelerated ship alone is capable of. The Campanile, taken over by daemonic forces and manipulated into Calth local space, is accelerated to its maximum with void-shields active. It manages to tear through several Ultramarine ships and the Calth orbital hub, decimating everything in the area as a prelude to the Word Bearer assault. Beautiful, breath-taking, terrible.
Also, the entire suicide run is described over a whole chapter. Actual time it took to cause all that destruction? About ten seconds.
Prior to all of the above, Abnett makes the anonymous narrator state a single line about the "Mark" of Calth - which refers to the solar radiation burns suffered by many of the loyalist soldiers who fought the battle. It will seem completely out of place given the terrible things that would unfold. But by the end of the novel, the reader will understand why the line was said. The reader will understand the simple, dignified truth of these words:
Secondly, the "mark" of Calth refers to the solar radiation burns suffered by many of the combatants, principally the human (specially non-transhuman) troops. The last of these veterans to die, many years later, still refuse graft repair and wear the mark proudly.
From Ventanus' and Selaton's desperate escape from the Word Bearers and their cultist hordes, to their defense of the Numinus Summer Palace, is simply awesome, especially given that both of them are aware that they are fighting a futile battle, simply to spite and wound the Word Bearers. The final, awesome piece of this. When they are reinforced by an Ultramarine Host, with a full array of speeders, main battle tanks and TWO Shadowsword Super Heavy armour pieces. The resulting fire fight cripples an entire Word Bearer and Cultist battalion, resulting in the main forces redirecting from their primary objectives, simply to crush this tiny point of resistance.
Death never looked so noble.
The last chapters of Know No Fear is one gigantic Heartwarming Moment for the Ultramarines, the Imperial Army, and the loyalist forces of the Mechanicum, culminating with this single line:
This is a dynamic combat shift. This is the game changed. Hesst would approve. Guilliman would approve.
This is preceded by a vanguard of Ultramarines, Mechanicus Skitarii, Imperial Army soldiers and assorted armour have taken a Data Engine that can retake control of the Calth orbital defense weapons. Unfortunately, they are being rapidly curbstomped by an entire warhost of Word Bearers. When Imperial forces, the scattered remnants of Ultramarine companies, Army battalions and a Titan armour unit begin returning the curb stomp in an unprecedentedly well co-ordinated assault that changes the estimated time of annihilation of the Ultramarines from three minutes, to well over an hour. And then the orbital weapons are returned to Ultramarine control.
A special shout out goes to Ultramarine Sergeant Aeonid Thiel, who single-handedly organized survivors on Guilliman's flagship armed them with the relic weapons from the Primarch's personal archive, and counter assaults the Word Bearers with a fury and tactical prowess normally seen from Captains. It should also be mentioned he is awaiting censure and discipline for running simulations and writing treatises on how to fight other Astartes. Thiel's censure markings are then repurposed- as the markings of Sergeants and section leaders for easy identification.
And how he also manages to defeat Sorot Tchure, a Word Bearer Captain, while fighting his two lieutenants at the same time. As Guilliman is showing Kor Phaeron his own black heart, Tchure, for a fractional second is distracted.
Thiel sees his opening, his practical. It is infinitesimal, a tiny chink in the Word Bearers guard. It lasts a microsecond, and will not be repeated. He puts his sword through it. The longsword shears the right side of Tchure's helm away.
Book 20: The Primarchs
Before that, in his novella Reflection Crack'd, Fulgrim gets quite a few moments of awesome for himself, not even counting besting a Greater Daemon of Slaanesh, a warp monster likely millenia old, in a battle of wills, and regaining control of his body. For starters, he one-shots a Titan by warping the flesh of its crew to swell so much it rip the machine apart from within. Then he shrugs off sound blasts that turn Terminators into a little wet smear. Keep in mind that all this is before Fulgrim becomes a Daemon Prince.
During The Lion short story in The Primarchs, Lion El'Jonson fights through dozens of Daemons which have managed to gain a foothold in the Invincible Reason after a Night Lords vessel pursuing it detonated their warp drive, causing the ship to be trapped in a warp rift. He ends the Daemonic incursion by fighting Kairos Fateweaver, and then defeats him by stabbing him in the heart, giving this incredibly Badass Boast:
Jonson: Did you see that coming?!
Book 21: Fear to Tread
The Blood Angels finally get unleashed in Fear To Tread, and it was worth the wait:
Sanguinius was one walking, talking Moment of Awesome in the whole book:
The Great Angel fully proves his reputation as "Most Badass Primarch" is justified during the battle in the Cathedral of the Mark: He kills two Greater Daemons of Chaos; Ka'Bandha, a Bloodthirster of Khorne, by tearing off one of his wings and throwing him back into a Warp gate, telling him "Only Angels may fly;" and Kyriss, a Keeper of Secrets of Slaanesh, by decapitating it while it's in the midst of a Villainous Breakdown. To emphasis this, Sanguinius is still recovering from both Ka'Bandha breaking his legs and a psychic backlash he felt from Ka'Bandha killing over five hundred Blood Angels with avsurdly powerful sorcery. On the tabletop, you traditionally need something approaching the power of a Baneblade to kill a Greater Daemon. An injured Sanguinius is as powerful as two Baneblades.
How he arrives at the final battle is pretty awesome: He flies through a window made of glass and bone, then, without even looking at it, throws his sword at Kyriss with such force it impales Kyriss in the wall while Sanguinius deals with Ka'Bandha.
He final Badass Boast to the Red Angel on when the Red Thirst should have taken him:
Sanguinius: But it did not. Because as long as one single Blood Angel lives, he will be the master of his spirit. He will not let the abyss that lies in the hearts of us all take him into darkness. That is the truth you did not understand, the truth that Horus has forgotten. It is not the descent toward the shadow nor the rise toward the light that makes us superior. It is the endless struggle between the two that greatness of character lies. We are tested, and we do not break. We will never fall! Take that to my brother and tell him!
Apothecary Meros: He sacrifices his life to take in the ragefire that is propelling the Blood Angels to madness, to insure that Sanguinius will not, but not before cutting out his own progenoid glands so that his gene-seed will live on. It does so in Rafen, protagonist of author James Swallow's Blood Angels series.
The Red Tear, the Blood Angels flagship, gets one: After Shipmistress Athena DuCade succumbs to madness and sets it one a collision course for Signus Prime and damages the controls, the Blood Angels are ready to abandon ship. Sanguinius, however, insists that the Red Tear, which has served the IX Legion since the first Expeditionary Fleets were launched, will survive. It's an extremely bumpy landing, and they almost die in the attempt, but the Red Tear manages to successfully crash land without exploding. When Sanguinius gives an order, no one defies him.
The Battle of Signus Prime: the Blood Angels are put through the wringer, and have lost more in that battle than all of the Great Crusade. However, they won. Not simply surviving, the Blood Angels actually deny Chaos any sort of meaningful victory. Erebus later rants at Horus that the whole thing was a complete failure and his fault, as nothing of importance was salvaged for the Chaos Gods. For the first time in the entire Horus Heresy, the Imperium won, and Chaos lost.
Horus, of all people, gains a CMOA during the epilogue: After Erebus goes on a tirade that Horus screwed up the Signus Prime trap by trying to get Ka'Bandha to kill Sanguinius and earn the displeasure of the Chaos Gods, Horus reacts with considerable restraint for a Chaos corrupted Primarch: the Warmaster calmly skins Erebus's face off, and tells the First Chaplain that the Chaos Gods are not the architects of the Horus Heresy:
Book 22: Shadows of Treachery
In Shadows of Treachery, the short story named Prince of Crows, we have Jago Sevatarion, First Captain of the Night Lords and Commander of the Atramentar, riding outside a Wraith-pattern starfighter IN SPACE where Night Lord and Dark Angel warships were exchanging fire in order to join his Primarch Konrad Cruze on the Dark Angel Flagship Invincible Reason. Keep in mind this feat is done when enemy ships are fighting each other in the void of space, and starfighters are effectively dogfighting each other.
Book 23: Angel Exterminatus
Nykoni Sharrokyn headshotting Fulgrim in Angel Exterminatus. A normal, if very skilled marine, making an extreme distance shot, inside an impregnable Iron Warriors fortress, surrounded by the larger parts of the Iron Warriors and Emperors Children legions, which he has managed to infiltrate. Made more impressive in that he uses a needle rifle; a fairly weak weapon to do so. And he escapes!
"Brother Sharrowkyn, is there something wrong with the floor?" And the moment this sentence is spoken, the awesomeness began as Sharrowkyn drops out of the ceiling and plunges TWO BLACK SWORDS INTO FABIUS'S CHEST, and then killing another noise marine by throwing one of his black swords across the medical room and embedding it into the noise marine's helmet.
Though that's nothing compared to what Sharrowkyn later does. In the three-way battle between the Eldar, the Traitor Marines and the Imperial Marines he battles Lucius who is pretty much the greatest non-Primarch swordsman alive and wins. But even more is that Lucius by this point has earned his ability to infect those who kill him and take over their bodies if they feel even an instant of emotion at his defeat. Sharrowkyn does not as he says Lucius is nothing but a rabid dog that needs to be put down, and he walks away having beaten the greatest swordsman in the Legions and doesn't feel a thing at it.
Marius Vairosean, now a Noise Marine, is blasted apart by Ignatius Numen...who, as a result of previous events, is completely deaf.
Perturabo earns numerous crowning moments in Angel Exterminatus, but special mention must be made of him smashing Fulgrim's face in because he's sick of Fulgrim's BS. For people who think Fulgrim's become a Gary Stu, it's awesome seeing the Emperor's Children Primarch taken down a peg.
Book 24: Betrayer
Betrayer has plenty of awesome moments in it, some that are crowning moments of awesome for the entire Heresy.
The Legio Audax, the Ember Wolves, are a Titan Legio that have about 90 Warhounds, the scout titans and while still potent, the weakest kind. So that is why it is immensely badass when about 40 of them assault the Imperator Titan Corinthian, a Titan that can annihilate cities in one shot, and use their secret weapons, the Ursus Claws, to bring it down. Each one blasts it's claws, which are basically harpoons, into the Corinthian and pin it down, then let the World Eaters on board to kill everyone inside it.
When a Warhound of Legio Oberon tries to kill Lorgar by stepping on him it is stopped just narrowly by Angron, who is actually standing underneath it's foot and is holding it up through sheer strength. A 45ft war machine exerting massive amounts of pressure, and Angron can keep it in the air.
The Conquerer and it's flag-captain Lotara Sarrin get plenty of awesome moments in the void battles.
Lotara Sarrin: "No one runs from the Conquerer".
Which then proceeds to fire it's Ursus Claws at two Ultramarine Battle-Barges, each one carrying thousands of Ultramarines on board, and then drag them back as they try to run, then tearing them both in half with it's lance cannons.
Roboute Guilliman's duel with Lorgar is epic, for both parties. Lorgar manges to crack Guilliman's skull, and Guilliman just shrugs it off. In the end, there is no winner, despite Lorgar being on his last legs and Guilliman looking ropey thanks to his skull injury: The battle is ended by the intervention of Angron, The Dreaded, the Red Angel, Primarch of the World Eaters and someone even Kharn the Betrayer and other primarchs run from.
Angron proceeds to show Guilliman just why people fear him. Even with his armour shattered, weapons blunted to the point of uselessness, suffering multiple wounds and his combat implants on the brink of killing him, he proceeds to beat Guilliman senseless. This is only made more awesome when, in response to Guilliman sneering at Angron, the primarch proceeds to deliver one of the best speeches of the series while driving him back:
"What would you know of struggle, Perfect Son? When have you fought against the mutilation of your mind? when have you had to do anything more than tally compliances and polish your armour?" [...] "The people of your world named you Great One. The people of mine called me Slave. Which one of us landed on a paradise of civilisation to be raised by a foster father, Roboute? Which one of us was given armies to lead after training in the halls of the Macraggian high-riders? Which one of us inherited a strong, cultured kingdom? And which one of us had to rise up against a kingdom with nothing but a horde of starving slaves? Which one of us was a child enslaved on a world of monsters, with his brain cut up by carving knives? Listen to your blue-clad wretches yelling of courage and honour, courage and honour, courage and honour. Do you even know the meaning of those words? Courage is fighting the kingdom which enslaves you, no matter that their armies outnumber yours by ten-thousand to one. You know nothing of courage. Honour is resisting a tyrant when all others suckle and grow fat on the hypocrisy he feeds them. You know nothing of honour."
Actually, I believe this is the Crowning Moment of Awesome to ADB. This only shows the depth of Angron, he was not the "Kill!Maim!Burn" character that many expected. He was strong, one of the mightiest, but he was also weak. Greatest warrior of the primarchs. However, he kept blaming others for the unfairness and the wrongs that affected his life. The Butcher's Nails that was installed in his brainn for instance. He also implanted these on his sons, in a complete act of hypocrisy. Not only that, but he seems to ignore how much the other primarchs struggled. Look at Sanguinius, born on a radioactive world with mutant cannibals. Mortarion, Kurze, Corax. Hell, even Guilliman who was born "lucky", Angron simply do not seem to acknowledge how much work was put into the Ultramar empire. Like there was nothing of importance or hardship in building and mantaining such achievement. The only thing the Red Angel did through his life? He blamed others:
‘You’re still a slave, Angron. Enslaved by your past, blind to the future. Too hateful to learn. Too spiteful to prosper.’
Elaborating a bit on ABD's work: many Black Library books are about making the main faction (A Space Marine Chapter, Imperial Guard battalion, or chaos warband) look cool while the opponents are just there to fold and make the home team look good. In Betrayer, all sides end up shining. Lorgar grows into a proper Magnificent Bastard whose plans encompass the universe and the future of Mankind; Angron becomes more than a raging beast and comes across as a liberator denied, a Spartacus-figure ruined by murderous brain implants who admits that he would have rebelled -sooner- had his mind been whole. Guilliman shows some verve in pursuing his brothers against all odds and holding his own against them for hours.
Aaron Dembski Bowden's description of just why people fear the Ultramarines - despite a massive advantage in men, numbers, and weaponry, and despite the story essentially being a narrative of defeat for the Ultramarines, Kharn and his cronies keep lamenting how they keep falling into Ultramarine traps and losing men for no gain. Even after Guilliman shows up, the Ultras are still outnumbered, and they manage to give much better than they get, despite ultimately losing. It's a meta-example as well - given how divided feelings about the Ultras are amongst the fandom, for ADB to write a story that shows them at their best, whilst dealing with one of their worst defeats, and all the while retaining the respect and adulation of much of the fandom, that takes some damn good writing.
Even more so the ADB's blog clarified the causalities of the battle the Ultramarines only lost a few hundred compare to the thousands the World Eaters lost, they're a reason why Horus wanted the Ultramarines taken out of the fight asap.
It took Angron the greatest warrior of the Primarchs and Lorgar to crush and make Guilliman retreat. Think about this, in the series we've seen primarchs die. Ferrus Manus being killed by Fulgrim for instance. Two primarchs engaged him (first Lorgar, crushing his head with the Illuminarium, while Guilliman power fisted his sternum. After that came Angron trampling the Ultramarines and jumping on Guilliman), and even so they COULDN'T KILL Roboute. He didn't win, hell Lorgar thought that he would be lucky to even be able to walk again(though Angron looked little better), but like Guilliman said on Calth: they were not trying hard enough.
Kharn delivering a very much deserved beatdown to Erebus, while using the fabled Gorechild, reforged for his hands. All in the name of brotherhood and vengeance for Argel Tal. And doing so while having a look of bored indulgence on his face.
Kharn: "Get. Up."
Betrayer finally answers the age old question of who would win in a fight: Angron or Leman Russ? And with the Night of the Wolf, where the Space Wolves and the World Eaters had it out over Angron's excesses and the Butcher's Nails, the answer is both would win in their own way. Angron proves that he is the stronger warrior, which is all that matters to him, but Leman Russ proved to be the better tactician, getting Angron encircled by Russ's Wolfguard while the World Eaters need for killing causes them to leave their Primarch to die. The only reason Russ let Angron live is in hopes that he would learn something from this, but Angron spends almost a century Completely Missing the Point before Lorgar, a man who despises being a general, explains it to him.
Book 25: Mark of Calth
Book 26: Vulkan Lives
In Vulkan Lives, the title character has a CMOA towards the end. When facing Konrad Curze in the Labyrinth, and seemingly denied the teleportation functions of his hammer, Dawnbreaker, he reminds Curze of one thing that he forgot about the weapon before proceeding to lay the smackdown on him:
Vulkan: "It's also a hammer."
Book 27: The Unremembered Empire
In "The Unremembered Empire" Curze singlehandedly infiltrates, butchers, slaughters, and generally causes wholesale destruction inside the Fortress of Hera on Macragge, one of the most secure worlds in Ultramar, whilst it's occupied by the greater part of the Ultramarines, Dark Angels, and other marine contingents from across the galaxy. And he almost kills Guilliman and the Lion, and technically Vulkan (several times).
Barthusa Narek, a Loyalist Word Bearer gives a Badass Speech combined with "The Reason You Suck" Speech to the two Possessed Word Bearers who help him reach Macragge.
Narek: I believe in the Word of our Primarch and I believe that Word makes us loyal to the Emperor. We are of the Word, and thus we are of the Emperor. It was ever thus. I despise the steps my Legion-kin have taken to embrace the Outer Dark. Too many steps, too far.
He then proceeds to blow Barbos Kha's head off with a series of bolt pistol shots and then kills Ulkas Tul with a sniper rifle at close range.
Tarasha Euten from The Unremembered Empire. A frail old woman who makes a rousing speech that spurs loyalist elements from at least six different legions to follow her command, who all end up shouting "We March for Macragge!". When it's later revealed that she is actually Guilliman's surrogate mother, it becomes clear where much of the Primarch's badassery comes from.
Some reviews have called it the "Anti-Horus Heresy" novel, as it highlight the unity and brotherhood of the loyalists, culminating in this scene:
It was odd to hear the cry uttered in a strong Chogoian accent, but in an instant the declaration was echoed with vigour by his fellow White Scars and then by every battle-brother in the hall. The war cry of Ultramar was coloured and invested by the accents of cold-hearted Medusa, of lofty Deliverance, of feral Fenris, of fire-forged Nocturne, of glacial Inwit, and distant, holy Terra.
How Badass is Tarasha Euten? She looked Konrad Curze in the eyes (something even Space Marines can't do) and told him to "Go to Hell". And this is just after Curze has utterly dismantled a Space Wolves pack.
Continuing a thread from Fear To Tread, a Space Wolves pack was dispatched to Macragge with the intention of watching Roboute Guilliman if he were to betray the Emperor in the aftermath of the Burning of Prospero. Guilliman had this to say about it:
Guilliman: Faffnar Bludbroder, do you really think that your pack can take me down? Faffnar: Perhaps not. You are Jarl Guilliman and your prowess is the stuff of legend. But we have our duty, and we would try. If you were, say, without your bodyguard and cornered in a room with us— Guilliman: My dear Faffnar, then you would be cornered in a room with me.
Book 28: Scars
During the space battle between the Alpha Legion and the White Scars, General Ilya Ravallion of the Departmento Munitorum, who has spent the time since the Ullanor Crusade trying to fix the White Scars logistical issues, complains that the White Scars are throwing away their tactical position. Her escort Halji says to keep watching. Then Jaghatai Khan gives a the command: as one, the entire White Scars Legion fleet shifts to combat speed (thanks to constant modifications to the engines which didn't get sanction from the Adeptus Mechanicus) and begins racing toward a weakpoint in the Alpha Legion blockade. The the acceleration is so fast that the artificial gravity on the V Legion flagship Stormsword needs a moment to compensate, sending Ilya flying into Halji, who explains that this is a standard tactic for the White Scars. The Alpha Legion is so blindsided by this (a genuine rarity for a Legion of Magnificent Bastards) that the White Scars break out of the blockade within minutes.
The Horus Heresy audio novels also deserve mention:
Garro: Oath of Moment
In Garro: Oath of Moment, we have this scene: "I am Nathaniel Garro, and I am a Legion of one."
Tylos Rubio, a Codicier of the Ultramarines Legion, essentially refuses to obey a direct order from Malcador the Sigilite, who is essentially the second most powerful person in the Imperium of Man after the Emperor.
Tylos Rubio: No, NO! You would have me leave the side of my sworn battle-brothers in their darkest hour? I refuse!
Nathaniel Garro: It is The Sigillite's command. His word is the Emperor's word.
Tylos Rubio: ...The Sigillite's command be damned. You give me no recourse, Garro. And so, on my honour as a son of Macragge, I choose to defy it. Even if it means I will perish here, even if you colour me a rebel like Lorgar's turncoats, I defy it!
Rubio and Nathaniel Garro, during the Battle of Calth
Garro: Legion of One
Garro: Sword of Truth
In Garro: Sword of Truth, we have Nathaniel Garro of the Death Guard Legion, now Malcador's Agent for the precursor Inquisition, dueling a Custodian Guard of the Emperor in a sword fight.