In 2157 a Citadel Council exploratory vessel is tasked with opening a dormant Relay into unknown regions of the galaxy. What they find is something they never expected. What happens when two civilizations centered around different technologies clash?

The most reviewed ''Franchise/{{Halo}}''/''Franchise/MassEffect'' crossover on fanfiction.net, [[http://www.fanfiction.net/s/7462757/1/Mass-Effect-Clash-of-Civilizations Mass Effect; Clash Of Civilizations]] by [=Eterna1Soldier=] describes the buildup and ongoing first contact between the humans and alien races of Halo and the alien races of Mass Effect.

Notable for its relatively slow pace, (Chapter 19 and diplomats from both sides are just ''now'' meeting each other for the first time) it is nonetheless an extremely popular fanfic.

!!This work contains examples of:

* AdaptationalHeroism: Saren. His {{Jerkass}} personality and bigotry is still present but the lack of both his canon FreudianExcuse and contact with Sovereign means that he's (possibly) a JerkWithAHeartOfGold rather than a KnightTemplar.
* AdventureArchaeologist: Liara.
** Technically, as someone who is helping with the expedition and analyzing the technology, Tali counts as well.
* AIIsACrapshoot: Both heavily subverted and played straight. The Geth are obviously hostile, but the UNSC AIs are friendly.
** ''Generally'' the UNSC AI's are friendly, AI's from Bungie-created universes run on rules that pretty much mean it's ''very possible'' for these AI's to go insane after a certain amount of time.
* AlternateUniverse: Aside from the obvious (the UNSC in place of the Systems Alliance, the Covenant species existing, the whole "Halo" side of the crossover), in this universe the Council lifted the ban on opening closed relays several years before the start of the story. In canon Mass Effect, this never happened, and even during the trilogy the law against opening relays was still upheld.
* BadassCrew: Shepard's Team.
* TheBattlestar: UNSC cruisers and Carriers. The ''Eternal Light of Sol'', a Forerunner ''Fortress''-class warship, takes it beyond [[{{UpToEleven}} Eleven.]]
* [[spoiler: BoldlyComing: {{Justified}}. The primary function of the melding is the reproduction. Also allows to quickly learn languages.]]
* [[CallARabbitASmeerp Call A Dog A "Shia":]] [[spoiler: Turns out Man's Best Friend has existed on Thessia just as long as he has on Earth. Asari keep them as pets, too.]]
* ConflictBall: Saren is in fine form in this story. While some of his antagonism is justified (he is in charge of security on the expedition, and he is right to be paranoid given the Rachni Wars), much of it seems to be for little reason whatsoever. On the other hand, see the JerkassHasAPoint entry below.
* EnergyWeapons: Played straight for the UNSC and Former Covenant. For species that use the Mass Effect though, KineticWeaponsAreJustBetter.
* ExactWords: Sergio Augusto said that the main armament of Sol was used on 'a barren, resource-poor planet'. He mentions that the weapon "atomised the entire planet," but omits the fact that it also destroyed every single Jiralhanae ship in its orbit. The implication being that it was a populated colony, [[KillEmAll perhaps densely so.]]
* FantasticRacism: In spades, especially against Tali.
* AFatherToHisMen: General Fealix.
* FirstContact: The premise of the first arc of the story.
** Ironically for the UNSC, the place where first contact is made? Harvest, the same planet where the human-Covenant war began. In fact, the whole thing starts out because the Citadel is examining the technology of the Tiara, as well as the battleships destroyed in the several battles for Harvest.
* FirstContactMath: Notably averted. Each side has their own method of deciphering language, neither of which involve mathematics.
* FirstContactTeam: The crew of the ''Hades Gate'' as well as Liara, Alenna, Saren, Mordin and Felix.
* [[GreenSkinnedSpaceBabe Hot Blue Alien Chicks]]: The asari, as [[SarcasmMode subtly]] pointed out by Ensign Duran.
* [[spoiler: HumanAliens: Turns out that the Asari and humanity are genetically related, meaning there's an actual reason for them looking like blue humans (other than "Blue Space Babes are hot.") Technically this applies to Shias and dogs, except it's more of a case of "Dog Aliens" instead of "Human Aliens" for those particular species...]]
* HumansThroughAlienEyes: The story is often told from the perspective of the Mass Effect races, although not exclusively.
* JerkassHasAPoint: Saren, ''repeatedly''. He's still an arrogant, racist bastard, but he takes his assignment as head of security seriously, makes valid points of observation, was correct in that Tali was sending illegal messages and even manages to prevent a potentially lethal situation from escalating by drawing his gun [[spoiler: on Captain Miles, giving the ODST a reason not to fire.]] Coupled with a willingness to sacrifice himself for the team makes it clear that this is ''not'' the same Saren confronted in ''VideoGame/MassEffect1'', whether or not he will grow into the same form remains to be seen...
* MentalFusion: The asari use their melding talents to quickly learn languages, and to help those they meld with learn ''their'' language. One of the UNSC's Huragok is able to tap into one of the meld-ee's neural jack, copy the Citadel languages, and provide everyone present with a translator.
* MileLongShip: Every UNSC ship above the weight of a light cruiser and every Covenant capital ship. Averted mostly with Mass Effect ships, as per canon. And like in canon, this is justified- Mass Effect fields on objects need increasingly huge power supplies the larger the object you're trying to have an effect on is. Ships larger than a kilometer (as in dreadnought-class) would require so much eezo that it would be impractical. UNSC and Covenant ships, which do not operate under eezo-based tech limits, circumvent this.
* MistakenForProfound:
--> (...)Liara (...) was utterly intrigued with the architecture. (...)Every now and then she would see words written on hanging signs, above doorways, and on walls in some strange alien language. Perhaps they were words of great wisdom or knowledge.
-->''''Restroom Ahead''''
-->''''Dining to the Left (Kids eat free!)''''
-->''''Do not spit over docking ledge. Thank you''''
** Averted when they find the statue of Sif. That actually ''is'' profound.
* NormalFishInATinyPond: Many of the technologies that the UNSC take for granted, such as artificial gravity, faster than light tech, and crystal computational devices, are utterly amazing to the Citadel Races, largely because they did this without any Element Zero, which was thought to be essential for the first two technologies, at least.
** That being said, the UNSC has no knowledge of Biotics, Mass Effect weapons, and many other advances the Citadel takes for granted. [[spoiler: This nearly causes the Citadel First Contact team to get slaughtered when one of the Asari tries melding with a human.]] Another thing the Citadel has that it considers normal that the UNSC doesn't is that they interact with many races of aliens who ''aren't'' trying to murder them regularly... Although it's somewhat implied that the UNSC is now on better terms with the former Covenant races.
* OOCIsSeriousBusiness: Hev, who is conservative even among its own kind, [[spoiler:after discovering the kinship between Asari and humans.]]
* PoweredArmor: The MJOLNIR Mark VIII, made from Forerunner metals that are lighter and stronger and it can be used by non-[=SPARTANs.=]
* ReasonableAuthorityFigure: Councilor Tevos has a far more realistic expectation of what possible contact will bring than her two colleagues.
* SchizoTech: The UNSC from the Council's perspective. They have space elevators, massive ships and incredibly advanced computing technology but still use projectile weapons with limited ammunition.
* SpaceElves: The Asari, but this isn't much changed from canon.
* StarfishAlien: The Engineers.
* StealthInSpace: UNSC ''Hades Gate'' can cloak itself, becoming both invisible to light and to other forms of detection. Though this was an established technology in-universe, as Halo: Reach and Halo: Legends proved.
* ThereIsNoKillLikeOverkill: The UNSC not only send in entire fleets into the neighbouring systems of Harvest, but also contact the Sangheili, who in turn send in a Supercarrier, which by itself is powerful enough to conquer a planet on its own. And to cap it off, the UNSC sends in the Light of Sol, a Forerunner Fortress Ship powerful enough to obliterate planets. All of this was a reaction to a small Turian patrol entering Harvest's orbit.
* [[Main/ThatDidntHappen This Conversation Never Happened:]] Miles, after discovered that Duran [[Main/WhatDidIDoLastNight probably]] slept with a female Sangheli.
* WhatCouldPossiblyGoWrong:
-->"And besides" Valern said, "even if they prove to be hostile to us, they don't even use element zero. How advanced can they be?"
* WrenchWench: Tali.