Tear Jerker / Shadow Raiders

• In the very first episode, Tekla helplessly sees her own planet consumed by The Beast, escaping through a teleportation gate, only because her royal guards, and her own parents performed a Heroic Sacrifice to grant her the chance. Then when she crash-lands on Planet Ice, the first two people she meets, Graveheart and King Cryos, don't believe her.
• In "Uneasy Hangs The Head", we discover the source of Graveheart's hesitance to assume a leadership role: His younger brother, Mica, died on the last mission Graveheart personally commanded as a quarrior. His guilt over failing to save Mica prompted him to retire from the military and instead become a humble miner.
• "This Is The Way The World Ends" is without any doubt the darkest and most tragic episode in the series. The Beast Planet attacks Planet Fire, and all efforts to stop it fail. The people of the planet are hastily evacuated as the gargantuan Eldritch Abomination draws near, and the World Engines that could've been used to move the planet away from it are discovered to have been sabotaged by Beast Drones, meaning that the planet cannot move quickly enough. After the evacuation is completed and everyone is watching the inevitable destruction of the planet, the Grand Vizier reveals that he is staying behind to die with his world, much to Pyrus' horror. The Vizier comforts the prince, telling him that he no-longer needs his guidance and that he chose his friends wisely, before plotting a suicidal ramming course with the Beast Planet. The attack has almost no effect, and Fire is devoured regardless, leaving Pyrus and his people as homeless refugees on the battle-moons.
• The entirety of the Planet Jungle episode. From the outset, Graveheart explicitly says their mission is to sacrifice another world, albeit one with no sentient life that they know of, to the beast to save their own asses. Everyone was gun-ho until this moment, and each member of the alliance slowly has doubts about how "justified" their cause was. Their guilt was absolved somewhat upon learning that the world had no working world engines, but at the same time they also realize there's absolutely nothing they could do to save it. Which only leads into the real tearjerker as the world's sacrifice did nothing to even slow the beast down.
• Throughout the Prison Planet arc, we see Sternum as A Father to His Men, working and fighting alongside those who follow him. At the end of "The Long Road Home", the Prison Planet finally returns to the Cluster directly in the path of the Beast Planet, which is just about to devour Rock. With no other choice, Sternum activates the Prison Planet's teleporter just as the Beast collides with it, warping them both away to parts unknown and likely spelling the demise of all of Sternum's True Companions in the prison.
  Sternum: "Are they dead? Are they safe? I truly do not know. In saving your family, your world, I have lost mine forever. All that I loved is gone."
• In "Rock and Ruin", Princess Tekla vainly attempts to convince a skeptical Lord Mantel of the danger of the Beast Planet and the need to work together for there to be any chance of survival:
  Tekla: Vox, show him the fate of Planet Water. To stop the Beast, the people of Water turned their entire world into one massive weapon. They fought with all they had and more, and in the end, it still devoured them. My world fell to the Beast soon after. The last of an entire planetary system...
  Mantel: So?! Their fate is no concern of mine!