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, approaching the twentieth chapter before diplomats from either side even ''meet'' each other, it is nonetheless an extremely popular fanfic.

!!This work contains examples of:

* AdaptationalHeroism: Saren. His {{Jerkass}} personality and bigotry against alien species is still present, but without Sovereign's influence he has not turned against the Citadel Council.
* AdventureArchaeologist:
** Liara, who begins the story working on an actual archaeological dig before being recruited to study the new alien species and its artifacts.
** Technically, as someone who is helping with the expedition and analyzing the technology, Tali counts as well.
* AIIsACrapshoot: Averted. Though the Geth remain a dangerous threat from the ''Mass Effect'' series, UNSC AIs are more stable and functional members of their society. They also possess the risk of going 'rampant', but are not inherently unstable.
* AlternateUniverse: Apart from the crossover nature of the fic, which removes the human Systems Alliance from the ''Mass Effect'' history completely, there are other divergences from the primary ''Mass Effect'' canon. The Batarians became a rogue state 200 years before the start of the series after conflict with the Asair, instead of only recently abandoning the Citadel conventions after conflict with humanity, and the Citadel Council has modified their laws regarding the activation of dormant Mass Relays.
* BadassCrew: Shepard's Team.
* TheBattlestar: UNSC cruisers and Carriers. The ''Eternal Light of Sol'', a Forerunner ''Fortress''-class warship, takes it beyond [[{{UpToEleven}} Eleven.]]
* BoldlyComing: [[DiscussedTrope Discussed]] by various members of the UNSC and the URS as they converse about various inter-species relationships. Also leads to some initial confusion when [[spoiler: Captain Alenna initiates a meld with Ensign Duran to learn his language. His confused attempts to explain what happened to other UNSC personnel sound like him saying they had just mated]].
* CallARabbitASmeerp: A creature almost indistinguishable from a dog is native to Thessia, known as a "shia", which the Asari likewise keep as pets. [[spoiler: They are not just similar to dogs, they ''are'' dogs (Or at least a close species relative).]]
* EnergyWeapons: Played straight for the UNSC and Former Covenant. For species that use the Mass Effect though, KineticWeaponsAreJustBetter.
* FantasticRacism: Saren is condescending and rude to Tali because of her Quarian heritage and dismisses the humans as a brutish and irredeemably violent species before they had even made first contact.
* AFatherToHisMen: General Fealix.
* FirstContact: The premise of the first arc of the story. Ironically, the contact occurs over Harvest, the same planet where the human-Covenant war began, as the Council forces study the technology and remains of the battles for control of the system.
* 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.
* ForWantOfANail: Though Tali and Liara become friends, without the authority figure of Commander Shepard to smooth over bigotry problems Tali feels resentment, bordering on outright hatred, towards the Citadel Council. [[spoiler: She circumvents Council law to surreptitiously send data to the Migrant Fleet, whereas in the main timeline she had enough faith in her superiors to ask for permission to do so, and would even abide by their refusal if they said 'no'.]]
* [[GreenSkinnedSpaceBabe Hot Blue Alien Chicks]]: The asari, as [[SarcasmMode subtly]] pointed out by Ensign Duran.
* HumansThroughAlienEyes: The story is often told from the perspective of the Mass Effect races, although not exclusively.
* HumanSubspecies: [[spoiler: It turns out that the Asari do not just look ''like'' humans, but are in fact a species relative. They, and a species native to Thessia known as a "shia", are genetic cousins of humanity and dogs, respectively.]]
* JerkassHasAPoint: Saren is an arrogant, racist bastard, but he takes his assignment as head of security seriously and makes valid points of observation. He is correct that [[spoiler: Tali is illegally transmitting data to the Migrant Fleet, and his armed standoff with ODST troops prevents a bloodbath after Captain Alenna makes a meld at the worst possible time]].
* 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.
* OOCIsSeriousBusiness: Hev, who is conservative even among its own oblivious-and-not-excitable 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.
* RubberForeheadAliens: Asari and humans are almost visually identical, with only a differing skin color and the presence or absence of hair. This is a huge surprise and shock to both species, as though they are each familiar with aliens of the same general bipedal shape, they have never encountered such an exact parallel before. [[spoiler:It turns out the Asari are a HumanSubspecies]].
* 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.
* ShownTheirWork: Ship sizes, technology and weapon effects are taken from ''Mass Effect'' and ''Halo'' wikis and official sources. [=Eterna1Soldier=] has pointed out specific sources when some fans have criticized the sometimes staggering size imbalance between the different universes.
* 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. 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?"
* WhatDidIDoLastNight: Ensign Duran is very adamant that he did not sleep with a Sangheli that one night....probably.....he thinks. Maybe.
* WrenchWench: Tali.