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Fanfic / XCOM The Third War

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XCOM: The Third War is a fanfic by TehArbitur53. It is a Mega Crossover featuring XCOM 2, RWBY, Red vs. Blue, Halo, and KonoSuba.

XCOM was not able to stop the Avatar Project from completion. Emboldened by their success, ADVENT expanded its sights and conquered alternate universes. Now, three years later, XCOM uses a newly acquired relic to summon heroes from these universes, so that they can finally defeat the Advent Coalition.


  • Adaptational Expansion: In this story, it's stated that, in addition to harnessing Earth's psychic power, the Templars are also interested in preserving Earth's culture. Their introduction to the story is when Geist contacts Bradford and the Commander to ask that they recover records of the music and plays held in the Sydney Opera House, in exchange for sending one of their own to assist XCOM.
  • Alternate Universe: Present in the backstory, as the Ethereals decided not to abandon Earth, which they were planning to do in War of the Chosen when they ascended, but instead decided to conquer the multiverse.
  • Benevolent Precursors: The Forerunners fill this roll, which is rather ironic given that they returned mankind to the stone age in their home universe: they not only provided XCOM with a means to summon heroes from other universes to aid them in defeating ADVENT, but also left easy to read instructions on how to do so.
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  • Chekhov's Gun: In Chapter 6, Megumin arrives alte to the mission briefing (she forced her way into the mission in place of Grif) and misses out on the fact that the Lost are attracted to loud noises, like explosions. Sure enough, she ends up using Explosion on a small group of Lost, attracting the entire horde to her team's location. Bradford concedes that they should have repeated that tidbit of information to her before she left.
  • Colony Drop: During ADVENT's invasion of Remnant, Amity Coliseum crashed into Vale.
  • Cruel Mercy: It's implied the reason ADVENT didn't destroy XCOM was so that they could live with the knowledge that they had failed to stop them. The Warlock, notably, becomes angry when they 'abuse' this mercy by rising against ADVENT once more.
  • Doomed Home Town: More like Doomed Home UNIVERSE. ADVENT has already conquered the universes the various crossover characters have come from, as well as many others. The reason most of our cast is still alive is that the device used to summon them pulled them in from a time before ADVENT arrived in their home universes.
    • To list a specific example, Beacon Academy from RWBY is shown to have been destroyed by ADVENT in the first chapter, in order to confirm XCOM's statements that ADVENT conquered their home universes.
  • Imperial Stormtrooper Marksmanship Academy: Darkness cannot hit anything with any ranged weapon, even managing to miss hitting a target with a grenade launcher. However, the aforementioned incident with the grenade launcher nocked a tree into several ADVENT troopers, taking them out. XCOM decides to make her a grenadier so that she stands a chance of hitting something.
    • Megumin also has shades of this, as seen in Chapter 7, but this is partially due to fear.
  • It's All My Fault: Megumin has a Downplayed case of this in chapter 7, believing she is responsible for causing all of the Lost in Sydney to attack the team. Notably, she's right: her usage of Explosion is what attracted the Lost to the team.
  • Loads and Loads of Characters: The cast includes twelve characters from RWBY, seven members of The Blood Gulch Reds and Blues, Master Chief and Cortana, the main cast of KonoSuba, and roughly ten or so from XCOM 2 and that number is only going to increase as the story goes on.
  • Mega Crossover: There's already characters from five different universes, and the author has expressed some interest in adding Titanfall to the mix. Additionally, Nagito Komaeda from Danganronpa makes an appearance in Chapter 4.
  • Mythology Gag: In Chapter Seven, Lost in Sydney, the corpse of a sectoid from the original game is discovered, with Julian noting that their corpses decompose very slowly.
  • Named by the Adaptation: Not really, as it was already in the game, but the author tacked on the names of the Chosen he fought in War of the Chosen to the ones that show up in this story.
  • Panty Thief: Kazuma's legendary luck once again allows his 'Steal' skill to net him a girl's panties, with Velvet being the unlucky girl in question.
  • Psychic Powers: Present in the setting, but XCOM doesn't have them at the moment - the psi-lab in the Avenger was damaged at the end of the Second War, and XCOM hasn't had the time to repair it. Geist offers to send one of his Templars to the Avenger to help fix the lab and begin training more psychics if XCOM completes a retrieval mission for him.
  • Rag Tag Bunch Of Misfits: Of the heroes summoned by XCOM, more than half are teens, and they aren't really that mature in personality terms.
  • The Remnant: Most of the Havens in this universe were destroyed, and by the time the story takes place, only three are left:
    • Mars: the largest of the havens, and functionally the center of the Resistance, it is located in the Himalayas. It's XCOM's headquarters. The Reaper's primary base is located nearby it.
    • Deimos: Located in the Sahara, it's built inside of a network of canyons and caves. Owing to this, its the primary source of raw materials for XCOM.
    • Phobos: Located on an island chain in the Pacific Ocean, it's considered too small and isolated to be a danger to ADVENT, and provides XCOM would food. It's also the headquarters of the Templars.
  • Sarcastic Devotee: Subverted with Julian - without the Restraining Bolt Shen put in his coding, he'd probably have tried to abandon XCOM a long time ago.
  • Spared by the Adaptation: Granted, we don't know if he actually died in the game, but here, it's explicitly confirmed that the Councilman is still alive. In fact, Chapter 3 is devoted entirely to his rescue.
    • The Chosen also count, as it was implied in the game that, should the Avatar Project be completed before they can capture the Commander, they would be executed for failing their mission. Here, not only are they still alive, but each of them was given an entire universe to rule over.
  • Story-Breaker Power: The author does not want to have characters that possess these, and has told a reviewer that he would not add in characters from My Hero Academia for this reason - they are way too powerful for this setting.
  • Word of God:
    • When asked how ADVENT managed to conquer the Halo universe, the author stated that they absorbed some of the Covenant remnants into themselves and used them to assist in conquering the UNSC and Swords of Sanghelios.
    • Additionally, in this timeline, the only DLC missions that were completed are those that were part of Shen's Last Gift
    • As for our heroes:
      • The cast from RWBY was pulled from a time between Volumes 2 and 3.
      • The Reds and Blues are from a time between Volume 13 and Volume 15.
      • Master Chief and Cortana are from shortly after Halo 3.
      • The KonoSuba cast were pulled from shortly after Season Two of the anime.

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