This will be an x-com/Mass effect crossover but only references to older species in both universes epically x-com but there will still be mass effect ones till a certain point so I can make a set up for the epic confrontation, this will be part of a time line to give you a small idea, almost all of X-com enemy unknown's ending has no involvement in this (exceptions There is a volunteer(no suicide) and a uber Ethereal.)

Two possibilities exist:

Either we are alone in the universe

or we are not

Both are equally terrifying

-Arthur C. Clarke

March 19th 2017: After two years of fighting the Ethereals and their proxy army the x-com's Alpha leader and another team (rest of alpha team recovering from injures) defeat the aliens but only alpha team leader Alex 'The Mage' Khan(Assault/Psionic with bad aim/aged 30 during the attack) survived with the rest being killed with the last one being killed by the Ethereal leader by ripping his body apart,giving Alex mental wounds as well as physical,after seeing this Alex used an unbelievably strong Psionic(not his newly learned rift but something more focused and stronger) attack that killing the Alien menace in 1 hit riding us of the menace once and for all... or so we thought(or for me the Commander like 5 seconds since so many ships escaped).

April 2nd 2017: The Avenger mk1 is built (as of present mk4s' are now the prominent avenger class with mk5 being tested) and sent to Mars (For many reasons such as colonisation using as stated by x-com scientists "A few Hundred or even a thousand years in the future alien technology" and to see if the aliens have been there and why they retreated they still had superior numbers and technology), because of Humanity's' lack of understanding of space travel (despite now being united and elerium (element 115) having FTL capability) It takes 4 days to reach Mars.

April 6th 2017: After arriving at the north pole of Mars for scouting the crew of the Avenger while in the new spacesuits made and tested before launch find a Ethereal base and find information and automated battle simulators(understood because of translators of both view and auditory which identify patterns which indicate certain words being used) for all the aliens they have fought (Including an ethereal type in between size and power of the 2 they have fought before!) and they also find an abandoned tank with the aliens' equivalent of a photo with the tank with a giant brain in it!, with 13 big Ethereals with one looking a lot like the one they killed on the temple ship, the base looks very old yet it has the same level of technology as the aliens that attacked earth with exception of the temple ship, using the data from the base humans advance another decade or so of technology (examples will be talked about later in the timeline.) Shortly after this with the base marked as an VIP (very important position) the crew head to the south pole finding a newer alien base but it seems to have nothing to do with the other base or the ethereals they find what would be classified as a Codex (Humans shortly after this create their own) with a lot of information but since whatever species who built this base now dubbed as the Protheans are less advanced than the also renamed EPPA (Ethereal Psionic Proxy Army) only navigation technology and other data which is useful when combined with EPPA data.

Humanity begins its long journey to become a galactic power but what will help or hinder them find out(some) next time so return soon for part 2 of timeline.

The other parts will be longer, first non timeline chapter will add something in between this and the may notice the huge difference change from mass effect's time line(wait till you see the effects) with morning war taking place about a bit over 120 years before the start of the timeline this number will increase by the name we arrive at the Citadel. Please tell me, my mistakes and also give me suggestions for a epic name for an alliance of species(I will tell you why later heh heh.) Also sorry for so many brackets.

Edit: I added the original quote because I fell it fits, I want to add the second one in the first non-timeline chapter and to tell you I will add the next chapter when Half life 3 is released... kidding, no it will be released around when I next feel appreciated by you people and I will tell you way it took, so the writer of the timeline is the commander who's name will be revealed in the codex of the third chapter along with age.