What do you think is the most terrifying apocalypse scenario?

As the thread title says. The zombie thread got me thinking. :P Personally I think it's the so-called Grey Goo scenario. tl;dr version: self-replicating nanomachines gone haywire and basically tearing everything apart at the molecular level. https://en.wikipedia.org/wiki/Grey_goo

"Please remain in your homes, if you are not at home, find shelter immediately. Close all blinds and shades, block out all windows. Do not look outside. Do not look at the sky. Do not make noise. Your cooperation is vital to your survival. Appointed government personnel will update you shortly." The light from my cell killed my eyes. This early in the morning what could be so goddamned important? As I read the emergency alert with one eye I rubbed the sleep out of the other. Then I sat up and read it again with both. And again. This has got to be some kind of joke. I rolled over and threw my legs off the side of the bed, working them into my slippers. I stretched and stood up, weakness wobbling my gait as I took a few steps towards the windows. I felt my heavy drapes resist parting, almost as if they knew what would happen. As I peeked out down the street, I didn't see anything unusual at first, then I remembered, the sky. I turned my gaze upwards to the sickly orange glow of the cities light pollution splayed across the clouds, and that's where I saw them. Small, black, jagged, like pieces of shattered onyx flocking through the sky, no wings, no sounds, countless shards of ebony wheeling and flitting to and fro in a chaotic scramble. And then I stopped breathing. My hands let go of the drapes, my legs fell out from under me. Laid down on my carpet, I couldn't move, couldn't breathe.. Oh God, why did I look? link to a voiced version of the broadcast- https://soundcloud.com/ammobyte/emperor_cartagias-emergency-alert

Death by a gigantic Meateor that can be seen from space. The hopelessness of watching approach and knowing there is nothing you can do. The moment where the atmosphere is burned away as you hear an ear shattering whistle. Then the wall of fire that approaches you from the horizon, and you hold your breath as it engulfs you. I've had that dream many times.

Arc -- Don't forget the all-crushing shockwave that would come before the wall of fire. :P And if anyone actually manages to survive all of that, they'd either suffocate from all the dust, freeze to death due the ash blocking out the sun or (in the best case) starve due to almost all plantlife dying due to the previous reason.

I think I will be dead when that happens. If not I will either find a way like a space ship or kill myself. I am not going to go though that.

Arc, I have had similar dreams before where the atmosphere is red or orange but never saw a meteor. Most times I have those dreams I am the only person left in the world.

It's the end of the world as we know it and I'm fine... We will kill ourselves or if we find another intelligent lifeform, they will kill us. Anything that comes here has a purpose and I highly doubt it's just to study life for the sake of knowledge. Life fights life to stay alive. Just look at what we did everytime we came across a new/old civilization. However, there are theories that the so called greys are just our future selves, so who knows?