I want to unknowingly help the big bad for a good amount of time. Have them build up to be our close ally (NO HINTS), have us honestly think they're a good person, and not just for a single patch, (unless it's an expansion release). Then they drop the bomb on us, and tons of people die. And we basically enabled it, maybe even helped them. Then we become hated, like REALLY hated for a long time too, and are relegated to being pariahs, trying to help people that hate us.

Oh, maybe toss in some power loss like when Midgardsormr suppressed our light, but something different, and we have to build it back up.

My after work, random on the spot, thoughts