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Tear Jerker: Horus Heresy
Bale Rane knew that death would probably hurt. War would probably hurt. Breaking up with your brand new bride and leaving her to go off to war, that would hurt too. Like a bastard. He never, ever, in a million light years, expected treachery to hurt so much.
The destruction of Monarchia and the humiliation of the Word Bearers Legion. Imagine Lorgar and his Legion, who loved the Emperor so much that he believed he was a god, even writing a book that argued that same point, and went from world to world proselytizing his religion, building grand cathedrals and inspiring masses of people. How did the Emperor respond to their efforts? By having the city of Monarchia (people included)one of the Word Bearer's crowning achievements, burned to ash by their rival Roboute Guilliman and his Ultramarines, and then forcing the entire legion to kneel in the ruins of the city, and then condeming the Word Bearers for their efforts, (efforts, by the way, had been allowed to occur for the better part of a century)note Technically speaking, The Ultramarines ordered the people to evacuate the city, gunning down those who resisted. However by the time the Word Bearers arrived to the planet only seven survivors were found.. and say that they all pretty much were failures, with the implication that they better straighten up or they would be purged from existence. Whatever your beliefs or philosophies, it's heartbreaking for a sincere devout son to be slapped down so hard. Seems that a lot of the Horus Heresy could have been avoided if the Emperor treated his Primarchs more like people instead of just soldiers.
The ending to Ben Counter's Horus Heresy novel Galaxy In Flames, where Saul Tarvitz stands with the surviving members of the Luna Wolves and loyalist Emperor's Children, having bled the traitor armies so badly that even Horus' incredible hubris has been broken, and Horus orders his fleet to bombard the Space Marines into oblivion rather than defeat them on the ground. Even faced with complete and certain annihilation, Tarvitz and his men won.
The scene where the virus bombs get dropped in Flight of the Eisenstein. Death Guard captain Ullis Temeter and Dreadnought Fal-Huron try to get their own troops to the shelters. Fal tells Temeter to get in as well, since his Dreadnought armor will protect him from the virus. Temeter tells him to eff off as he shoves one last marine into the shelter, then locks the door. As the bombs drop, Temeter realizes that Fal has a couple of cracks in his armor that means the virus will still get him anyway, and the two exchange "you too, huh" one-liners before dying. Made worse by the fact that the defining trait of the Death Guard is their resilience to toxins and disease. Hell, the fate of the entire Death Guard legion could qualify - the traitors got lost in the warp and caught by Nurgle, who made mockery of their vaunted immune systems. The loyalists become the Inquisition, arguably one of the reasons why life in the Imperium sucks as much as it does.
Three simple words; "We are betrayed"
In Galaxy In Flames, the last parting of Loken and Tarvitz. Tarvitz says that it may be they will not meet again; Loken says he thinks there's no maybe about it. . . and then, in the scene described above. Tarvitz had desperately gotten to the Emperor's Children so that he could die with his brothers, in defiance of the breaking of brotherhood that Horus had imposed on them, but at the end, he looks about the survivors — Emperor's Children, Luna Wolves, World-Eaters — and realizes that he knows all their names, and that men who had been only faces to him had become his brothers.
On the Warhammer 40K note, the ending of the Horus Heresy novel Descent of Angels where Zahariel, the hero of the Dark Angels first battle as a legion (not to mention almost 413 pages of story), is sent back to Caliban with Luther, while his cousin Nemiel stays with the main force because their Primarch distrusts psykers. Anyone who knows their fluff knows what happened at Caliban...
As of "The Primarchs" novella "The Lion", that nightmare scenario has been averted, but in the absolute worst way possible: During his time in the Crusade, Nemiel became a Brother-Redemptor (predecessor the Dark Angels Interrogator-Chaplains in 40k) charged with upholding the Edict of Nikaea banning Librarians. When Lion El'Jonson's flagship gets pulled into a warp-rift allowing Daemons to invade the Invincible Reason, Jonson decides to revoke the Edict of Nikaea after one former-Librarian breaks the Edict and uses his powers to effortlessly destroy one of the Daemons. Nemiel is oath-bound to prevent this, and looks set to fight his own Primarch, when Jonson suddenly lashes out and behead Nemiel with a single swipe. So when Lion El'Jonson finally returns to Caliban, there's little doubt that Zahariel will learn that his cousin was killed by Jonson, and it's doubtful that Zahariel will be feeling much loyalty to his Primarch...
The short story After De'shea, where Angron is so furious over being taken from his band of rebel gladiators he can't properly form a coherent sentence. You know that this guy goes traitor and becomes the leader of the most psychotic group of warriors in an army of psychopaths, but when he's struggling with his loss over his friends, his brothers and sisters, you don't see the Daemon Prince who will burn seventy sectors or doom Armageddon...you see someone who just lost the only family he ever knew. It's really sad.
Deaths of Kiron and Gythua in Outcast Dead.
Deaths of Phosis T'kar, Auramagma, Uthizzar and many of the Thousand Sons in A Thousand Sons.
T'kar's death is one of the hardest. He butchers his way through a horde of Space Wolves and Sisters of Silence, finally reaching the legendary Constantin Valdor who calls him a monster. T'kar sees his reflection in Valdor's armour and realises he has mutated, very extremely. Before he dies he closes his eyes, sheds a tear and responds to Valdor's insult with "I know."
When Ahriman is told of this he realises that he did infact consider T'kar a friend, and he is hurt that it took his death to realise that.
A large part of A Thousand Sons qualify as a tearjerker, considering it deals with the fall of one of most loyal legions. From what we see not only did the Thousand Sons comprise of some of the best and most open minded scholars in the whole Imperium, most of them were truly nice and polite and their homeworld a veritable paradise. Too bad, that they became unwitting pawns of Tzentech.
Most of the scenes involving Argel Tal in The First Heretic, particularly straight before he orders that the Word Bearers fire upon the loyalists on Istvaan.
The epilogue deserves a special mention: Argel Tal is sitting in his quarters looking at the data slate that Cyrene was writing when she was killed. Following this, he looks at Aquillon's sword, the very weapon which killed Cyrene. Argel Tal and the Gal Vorbak then killed Aquillon and his fellow Custodes, but not before Xaphen and several other Gal Vorbak are killed. Looking at these memories of the three closest people in his life, Argel Tal reflects that all his brothers are dead. And then comes the absolute bitterest knife twist in the book:
Solomon Demeter in Fulgrim. He is defending the Precentor's Palace and finds his fellow Istvaan loyalist Lucius fighting a squad of Emperor's Children marines. Solomon rushes in to kill them, which the two of them do easily. Only once they are dead does he realise that they too were Istvaan loyalists! Lucius is the real traitor. Before he can do anything Lucius fatally wounds him and declares Solomon, Tarvitz and all the other loyalists to be fools, and that he will not die with them. Solomon sheds a single tear as he dies and sees the skies begin to rain.
In Know No Fear, Venatus realises that the Mechanicus server has blood on her sleeves, realising that someone died, cradled in her arms.
Later, Tawren reveals the identity of the man who died in her arms with the following words:
He was, I suppose, my husband. My life partner. The Mechanicum does not think in such old-fashioned terms, and our social connections are more subtle. But yes, captain, we were close. A binary form. I miss him. I do this for him.
Also, Magos Uldort. She volunteered to remain at a command center that was about to be overrun by the enemy, fully aware that it was a death sentence. She nonetheless performed her duties diligently and directed all possible reinforcements to help Tawren and Venantus. Because of her, Tawren was able to retake control of the Calth defense grid, which in turn saved the remaining Ultramarines and their Primarch. The reader never sees her final moments, nor will they ever see her speak a single line. But the bittersweet triumph at Calth will ultimately be owed to her.
Hell, the entire plotline of Prospero Burns and A Thousand Sons. Two brothers, almost polar opposites, but on the same side; one tries desperately to warn their father of an impending disaster, the other begs, even as he is on the way to kill his sibling, for the other to stand down and set aside what he sees as foolish heresy. Both brothers genuinely just want the best for the other. But Daemons, distrust and plain old terrible luck conspire to see the destruction of the two most deadly threats to Horus. Just to emphasise: Leman Russ and Magnus The Red disliked, distrusted and loved each other.
Fulgrim's My God, What Have I Done? moment in Fulgrim. Right after killing Ferrus Mannus, his most beloved brother Primarch, Fulgrim realizes that everything that he took as a slight towards him from Ferrus were actually well-meant jests, and that he had doomed his Legion to heresy the moment he had taken the blade from the Laer temple.
Moments like that run throughout the Horus Heresy, and it makes the whole thing much sadder when you realize that a lot of the pain and bloodshed of the heresy was completely unnecessary.
And then the absolute tragedy of the event: Standing over Ferrus Manus' body, Fulgrim realizes what he and the Emperor's Children have become, and begs the Daemon in his sword to end his pain. The Daemon proceeds to possess Fulgrim, forcing his mind into a painting of Fulgrim on the Pride of the Emperor. For the better part of five years the fandom assumes that the Daemon was responsible for Fulgrim's subsequent atrocities, until The Primarchs novella The Reflection Crack'd, where it's learned that Fulgrim was able to forcibly swap places with the Daemon, but the effort drove him completely into the embrace of Slaanesh, and now doesn't regret killing Ferrus Manus. What 40K has been telling us for years was proven true in the tragedy of Fulgrim: No one who has touched Chaos can ever escape it.
The confrontation between Sanguinius and Alotros in the beginning of Fear To Tread. Alotros has succumbed to the Red Thirst, which unlike the Black Rage predates the Horus Heresy and has slowly begun to emerge among the Blood Angels Legion. Alotros is drinking the blood of fallen Nephilim, and Sanguinius tries to reason with him, hoping against hope that he can bring his gene-son from the depths of insanity. Alotros stares at his Primarch for moment, then attacks him, forcing Sanguinius to kill him in self-defense just as much as it was giving Alotros the Emperor's Peace. For years fans have speculated that Sanguinius's presence might be able to bring Blood Angels back from the depths of the Flaw. As of Fear To Tread, that theory is Jossed with extreme prejudice.
Really, the entire prologue, showing Horus and Sanguinius talking as loving brothers, with Sanguinius even trusting Horus enough to tell him about the flaw in the Blood Angels legion, and confiding that he's afraid his legion is going to be wiped from Imperial history. It really shows that these two brothers genuinely loved each other. Those who know their 40k backstory know that Horus kills Sanguinius, and he doesn't make it quick or painless.
The fate of Meros is also pretty damn depressing even by 40K standards. Most Heroic Sacrifices are at least merciful enough to kill you. Meros's, however...death would probably be preferable.
His actual dreams are equally sad. His ideal world is one where he's designed beautiful, functional cities that are the centre of culture, learning, and every other civilised virtue. He's not even fussed about being the ruler of it. And he knows that it will never ever come to pass.
When Guilliman finally catches up with Lorgar in Betrayer. The look of total, all consuming hatred on Roboute's face makes Lorgar realize that Guilliman never despised him the way Lorgar had always assumed he had. It's not quite a My God, What Have I Done? moment, but it speaks volumes that Lorgar feels ashamed that so many of his actions have been led by an entirely false opinion of his brother, and feels a need to try and justify his actions.
While a character's death almost always gives the reader some sorrow, Argel Tal's was particularly heartbreaking. Having one of the most compassionate, and arguably most human, characters in the series die such a way hit pretty hard; the fact that everyone thought he was going to have a more dignified death (since his death was a Foregone Conclusion) did not help.
The state Vulkan is in after Vulkan Lives. Captured by Konrad Curze, it was discovered during his torture that Vulkan was a Perpetual, a being who cannot die. Enraged that he couldn't kill him, Curze repeatedly killed Vulkan, destroying his mind. By the time he managed to find his hammer Dawnbringer, which had a built in teleporter, he was devolving into a raving maniac. The teleport function allowed him to escape the Nightfall, but he teleported into Macragge's atmosphere and burned up on reentry, and survived again. Unfortunately, when discovered by the Ultramarines in The Unremembered Empire, the constant deaths had driven Vulkan completely insane (even by 40k's broad definition of the word) trying to attack everyone he saw. The Cabal, the group responsible for the Alpha Legion's defection to Horus, wanted Vulkan killed to prevent him from returning to Terra and helping the Emperor, but Eldrad Ulthran told John Grammaticus that by using a fulgurite spear imbued with the Emperor's psychic lighting could heal Vulkan's mind instead of kill him. Grammaticus stabbed Vulkan, but it failed and Vulkan died. We get a brief Hope Spot where Guilliman and a group of Salamanders maintain that Vulkan merely needs time to heal (one Salamander thinks he heard a heartbeat) until we learn the name of the preservation capsule containing Vulkan's body: The Unbound Flame, the last and most mysterious of the Artefacts of Vulkan that the Salamanders believe their Primarch left for them to find when he departed.
The Wolf of Ash and Fire
‘I was there,’ he would say, right up until the day he died, after which he spoke only infrequently. ‘I was there the day Horus saved the Emperor.’
This entire story is a Tear Jerker. The Emperor and Horus had such a good relationship with each other, and at the end of the story they set course for the place where it all begins to go wrong. Ullanor. It's heartbreakingly sad seeing these two so close, knowing that soon they'll be enemies and that the Horus we're seeing now is soon going to be gone and replaced by the evil Warmaster Horus.
It's also a Tear Jerker because we get to see all the familiar faces of the Luna Wolves, before the Nightmare Fuel that was Istvaan III. Tarik Torgaddon and Hastur Sejanus joking with each other over strategy, Horus Aximand being the voice of reason, and Abaddon striving to make his Primarch proud. They'll never be like that again...