So uh... Why do in game over alien transport Earth to their dimension? Did they just need new planet or what?
If you check out X-Com Interceptor, the odd prequel, you'll understand the details. The final level have the Alien Empire build a superweapon behind a black hole in a pocket universe with a solar system. You send a fleet through the black hole and do a desperate run against the planet-sized monstrosity of a desperately built superweapon and send a star breaking bomb to trigger a supernova in the star behind it. The solar system had a planet with life on it that "didn't correspond to that of the Aliens". Come Apocalypse we see a planet likely suffering from a heavy Gamma-Ray burst and in ruins. Likely the alien organism who was minding its own business found our universe and is bugfuck mad about being roasted by a dead star.
Their own planet was dying. So yeah, they needed a new planet. Earth just happened to be good choice. Well, compared to their world.
This troper is disinclined to believe that animation. I've always thought that the aliens in that game were an entity similar to the Combine, with the Micronoids being the only real constant in a highly diverse set of universes and species. The design of the city suggests that it's an outpost for an empire rather than a struggling city barely eking out an existence.
This is confirmed by the game - the aliens cannot survive on Earth, their life cycle aside, they are largely sterile, short lived, and have a predisposition to degenerating in the atmosphere. They are also host to a species of sentient mind-controlling super virus which is the driving force behind their invasion plan and what will actually live on earth when they're done.
And how the heck you capture Overspawn? Apparently, its possible in the normal game...
There is currently no known way to capture it. It has UF Opedia entry for alive and pic, but capturing it is considered impossible. This troper thinks it was dummied out, since it is considered as "vehicle" by the game, not as alien. Also, in my knowledge, no-one has captured it alive without hacking the game.
It could also be just part of what they did with the programming. In order to have a 'autopsy', they have to have a 'live capture' as well. And in case of a glitch occurring and somehow someone gets the live capture, it simply gives them the text and continues on, no need to crash the game because 'live capture not found'.
This makes a lot of sense, especially considering there was such an oversight in UFO Defense, it was possible for corrupted save data to create a research topic for the Zombie alien type, which caused the game to crash when completed as Zombies were never meant to be capturable, and thus never had a research result programmed in.
The original intent was that you needed to research a facility called Advanced Alien Containment, then collect samples of Overspawn tissue in a special set of missions in order to get the Ufopedia entry for the live Overspawn. None of this made it into the final version of the game. As is, it is indeed impossible to capture an Overspawn.
The Aliens only need the planet, in whatever shape it is. And the city the game takes place in is supposed to be the last inhabitable earth city, while everyone else lives offworld. This brings the question: Have the aliens never heard of "diplomacy"? They can inhabit the rest of the planet, or failing that, humans can live out of it. This could all be solved if they just approached X-COM or whatever governing body the city has and request asylum or something.
You're assuming that diplomacy is easy. In the case of just human diplomatic relations, in history, they've not gone well for something as obvious as not speaking the same language. If you factor in religious, economic, and political difference, diplomacy is complicated and time-consuming. Now consider the aliens: they have nothing in common with humanity, and nowhere to go to get common ground, and not even the beginnings of a linguistic translation ability. Sure, they could have tried diplomacy, but they were also on a time limit, and probably figured that they could just beat the crap out of these puny humans and take over in short order.
That and Humanity just got done with Two Separate Wars against aliens, the last of which made earth the mess it is. Cult of Sirius aside, I don't think most humans would be in the mood to negotiate with a bunch of aliens who shot first.
