Mass Effect 1
We had assumed Saren was using the powerful, ancient ship Sovereign to level up his evildoing stat, not the other way around. But when the ship deigns to hold a condescending conversation with Shepard, the truth comes out: it's not just a Reaper ship—it is a Reaper.
This is the first real glimpse at what the Reapers are capable of. And unless Sovereign is bluffing, the galaxy is pretty screwed. "We impose order on the chaos of organic evolution," he says. "You exist because we allow it. And you will end because we demand it." It feels really good to kill this thing later on.