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And the final level had THREE unsaveable separate levels full of extreme ambush points that had to be completed with no failure, a textbook example of "oldschool gaming". Oh, and did we mention that the only way to research the best armour in the game was to capture a live specimen of a rarely-appearing alien? And that to capture a live specimen you had to send a poor redshirt in armed with an oversized cattle prod to stun it in close quarters? And that said alien was able to tear a human apart effortlessly in close quarters? And (as if all that wasn't bad enough already), that the poor redshirt then had to pick up and to carry the damn unconscious thing back to the human ship praying that the specimen didn't wake up with a grudge before he got there and the mission ended? Yeah, it was kinda like that. Not only do aliens have a major advantage technology-wise at the start and they tear through your recruits with impunity, but later in the game when you catch up they start using a nasty trick to take control of your troopers, leading to sometimes hilarious and extreme rage-inducing turn one TPKs as your heavy weapon trooper gets controlled and fires his heavy weapon inside the transport while the team is busy disembarking. X-Com now again must protect the people while finding out about where exactly T'leth is and go in to blast it to smithereens with extreme prejudice.
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Story-wise, the destruction of the Martian base activated a fail-safe in the aliens' system that woke up T'leth (not R'lyeh, but you get the point), their 'secondary' (and mobile) base buried deep under the seas of Earth, ran by a "Sleeping, Gigantic Alien In Coma That Can Manipulate Dreams" with an octopoid face, real subtle there. You do the math) and infinitely harder with a direct nod to Call of Cthulhu.
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It was good enough to spawn a sequel, Terror From The Deep, which was basically the same game, but underwater (therefore areas to cover are multiplied by three. You didn't exactly have a time limit, but the more time passed, the harder aliens arrived, starting with one-man(alien?) scouts to do Roswell-grade pranks to Harvester grade ships to abduct and mutilate cattle, to fuckhuge battleships that eventually started attacking your base.
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It falls upon the Extraterrestrial Combat Unit, or the eponymous X-COM, a multi-national task-force, to deal with the invasion by blasting the fuck out of the Aliens, stealing their shit, researching their technology, and using it to fucking kill them off, all the while fighting off terror attacks and trying to figure out where in Uranus these fucking things are coming from (as it turns out, from Mars, and if you win, the final mission involves you going to Mars and dropping an Exterminatus on their asses). Aliens are arriving, and they are most assuredly hostile.
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