6. Xenosaga Episode I: Der Wille zur Macht (2003, PlayStation 2)
One of the freakiest alien invasions in game history comes in the overblown epic Xenosaga. The first several hours of the cutscene-heavy RPG detail the crisis brought on by the appearance of the Gnosis. These disturbing, malformed aliens do most malicious space races one better - they can't even be touched by conventional weapons. The first several hours of the game is consumed by a tense battle as the Gnosis engulf - and, eventually, destroy - the research ship Woglinde.
Though the Gnosis are completely impervious to guns, and use their bizarre powers to crystallize and shatter human beings, the most disturbing thing about the invasion is how the mask of sanity on the embattled soldiers cracks. Fortunately, badass, beautiful android KOS-MOS can take on the menace. Don't hope to catch your breath, though: the Gnosis keep on coming for the rest of the three-game series.