If you know what a "Rachni" is, then congratulations: your entire life has been building to this moment.
Speaking with PC Gamer, BioWare's Casey Hudson gave a rundown of how your choices from ME 1 and 2 will affect the twists and turns Mass Effect 3 takes en route to its massively effecting finale.
"I think a way to think about it is if you made decisions early on, you’ll see them affecting this," he said. "And the decisions you might want to make that go against those prior things are gonna be harder. Killing the Rachni might present opportunities in Mass Effect 3 that you wouldn’t otherwise have, but if you don’t take those opportunities and you try and do something in opposition to that, then it would be harder for you than if you work with it."
He described the process as being focused on "greater and greater degrees of success" - as opposed to brutally punishing players for unknowingly making a bad call early on. The game's ending, then, will be an extension of that mentality.
"If you just rip straight down the critical path and try and finish the game as soon as you can, and do very little optional or side stuff, then you can finish the game. You can have some kind of ending and victory, but it’ll be a lot more brutal and minimal relative to if you do a lot of stuff. If you really build a lot of stuff and bring people to your side and rally the entire galaxy around you, and you come into the end game with that, then you’ll get an amazing, very definitive ending," Hudson explained.
So then, a Rachni-mounted Krogan army with endorsements from every store on the Citadel atop a man-made Reaper named "Steve" who's been painted to look like a billowing American flag? If that's not definitive, we don't know what is.