What is it that makes aliens such a threatening opponent in humanitys mind? Is it the fear of the unknown? Is it the likelihood that whatever invading extra-terrestrial force we might encounter will be more technologically advanced than us? Whatever aliens appeal, theyve been an ever-present in pop culture far longer than any of us can remember, so its unsurprising theyre prominent fodder in video games. Heres a selection of 15 excellent single player games where you face off against interstellar forces.Mass Effect TrilogyThe much-acclaimed Mass Effect trilogy needs no introduction, these action RPGs ushering a new standard for narrative-heavy third person shooting that other space-faring shooters have struggled to match. The series central antagonist are the Reapers, a race of sentient star-ships who originated one-billion years prior when the galaxy controlling Leviathans created the synthetic lifeform the Catalyst which then turned on its creators, absorbing their genetic material to create the Reapers. The Reapers awaken every 50,000 years to conduct galactic-level mass destruction with the trilogy centring on Commander Shepard and his crew stopping them.