Tear Jerker / The Roboutian Heresy

• The entire Heresy, period. Especially with revelation that it was caused by Emperor's Shadow, Yellow King. Just like in canon, manipulations of Chaos, lies of the Emperor and flaws of Primarchs caused a horrific disaster.
• Konrad Curze's foster mother taking the blow meant for him.

  "You are not a monster."

• Sanguinius threefold fall to Chaos is probably the saddest of any of the traitor Primarchs, even compared to canon. When it comes down to it, he falls because he's not strong enough to face the consequences of his actions, and would rather live a lie that he was still the righteous angel, than face his mistakes.
  Something broke within Sanguinius. He couldn't bear it anymore. He had thought that he was strong once; that he could face the truth and carry on. He had been wrong. He was weak. He had fallen, and now all that remained was to fall even further down. The Thirst was too strong, the horror too great. He wanted it to end, but he knew, without knowing how, that death would not release him. There was only one way for him to be free …
  • Subverted HARD with the Angel War's revelation that he was faking his delusion. In reality, he has long since devolved into a narcisistic monster who has completely accepted the taint of Slaanesh.
• A dying Barabas Dantioch sacrificing himself at the Dark Glass station to teleport the World Eaters' and Word Bearers' fleet away from the Ruinstorm.

  "Please, tell my father that in the end... I died with dignity."

• Lucius the Reborn's entire story arc was pretty tear jerking.
  • For context, Lucius was very much a selfish bastard in canon, switching sides on Istvaan III etc. Here he's captured by the Dark Eldar with the rest of thr Emperor's Children and tortured until his hearts give out without touching his handsome face as part of a sick joke. Saul Travitz, his best friend, attempts to revive him to the mocking laughter of the Dark Eldar. He succeeds to everyone's shock and the Dark Eldar take him away to be tortured, the last thing Lucius ever hears from Saul is "You must live Lucius,Live and take revenge" Saul is later described to be tortured to death, without making a sound as his final act of defiance. Lucius finds a fraction of a scalpel and scars his face to remember all his fallen brothers
  • Eventually the Emperor's Children escape with help from the Night Lords and go to Terra to relieve the defenders. It's here where Lucius enters the history of the Imperium; He's reported to have fought side by side with Abaddon and gave his life to save Abaddon from death. Reports also say he did the same thing for Ahriman, Garro and whole host of other loyal heroes.
  • In the story excerpt, Lucius is narrated as having seen the ghosts of his friends, leading him and pointing to who needed saving. As he stands on his latest kill, a Warhound Titan, he wonders if he's finally done enough and if he can finally rest with his brothers. As the Titan's reactor explodes, everything goes white and he doesn't wake up anymore.
  • And to cap it all off he's resurrected after the Emperor dies, and forced to put down his brothers in the legion after they have been corrupted.
• Fulgrim, when rescued by Sevatar, Rylanor, Vespasian and co. from the clutches of the Dark Eldar:

  "S-s-sevatarrrr ... Whe-where isss Konrradd ? Wherrre iss my bro-brotherrrr ?"

  • Ten thousand years later, Rylanor is killed by Fabius' Fulgrim clone.
    'Fulgrim ?' said Rylanor hesitantly, knowing even as he spoke that it wasn't. 'Is that you, father ?'
• Perturabo's musings after the Heresy. He sees nothing in the future but endless war and misery, but that he cannot allow himself to dwell on it because his sons need him to be strong. He sounds so defeated.
• Mortarion's musings during the Heresy on betrayal, on how unthinkable it was, and on how it is effective precisely because betrayal is unthinkable to its intended targets.

  If treachery did not hurt so much, it wouldn't be nearly as effective. If evil wasn't so unthinkable, it wouldn't be evil ...'

• The fact that Lion'El Johnson, Daemon Primarch of Tzeentch, does regret his treachery deep down (if only by Tzeentch' ploy) and was close to accept redemption offered by Lorgar but rejected because he crossed Despair Event Horizon, is sad, despite everything he has done.
• The Emperor's death in chapter 40, refusing to ascend to godhood in order to fulfill his duty and give humanity one final chance to survive the Times of Ending.

  "None of us ever get what we want. But we do what we must, not what we want. That is who we are. That is what sets us apart from our enemies. Remember that, always."

  • Omegon's reaction to the above. For ten thousand years, he was the only Primarch left, and still he kept going because he hoped making his father into a god would finally solve everything. Then his own father crushes this hope, because he doesn't want godhood and believes Him not becoming a god is the only way humans could win the Times of Ending. Poor Omegon can just sob he's not strong enough to keep going this way.
• Jurgen dies in "The Cadian Apocalypse Part III", sacrificing his life to save Commissar-Castellan Cain from a Dark Angel marine. Also, the tragic fate of the Lord of Wraiths and his regiment of undead guardsmen-turned-vengeful-wraiths.
• The Doom of Eisenhorn. Gregor turns out to be a vessel for Yellow King, the Enemy Without of the Emperor, and all this time his friend, Pontius, was looking for him to prevent this, which he has failed.
  • The events which led to its creation: Past Emperor and Ollanius found the Tower of Babel, which was a building where geniuses were put into wall to uncover secrets of enuncia. Emperor wanted to preserve it and use it to protect humanity, but Ollanius was against it, as its nature and preceeding horrible misuse of enuncia would definitely lead his friend to ignoble path. Though The Emperor came to agree with him and the tower was destroyed, the choice has already been made, and it became a seed for Yellow King.
  • The identity of Voice, the Herald of Yellow King? It's Gideon Ravenor.
• Farsight's backstory. He, Shadowsun, and Kais all trained under puretide, just as in canon, and were bonded in the Taliserra ritual (sort of a mix between blood-brothers and pseudo-marriage), until Kais went renegade and murdered Puretide, forcing him and Shadowsun to hunt him down and kill him. Except that's not what happened; the Ethereals killed Puretide in a botched experiment to replicate his knowledge, and Kais was the only one who could resist their mind control, smash the engram device (saving Farsight and Shadowsun in the process) and flee. The enraged Etherals brainwash Farsight and Shadowsun into believing Kais was a murderous traitor, and sent them to hunt him down. He died crying as his best friends cut his throat, unable to hear him through the mind control. When the Dawn Blade reveals this to Farsight, his reaction is a mixture of My God, What Have I Done? and raw determination to become The Atoner.