A giant solar flare disables all airliners currently in the air. They all crash. Millions die in the space of a few minutes. Together with that, there is a magnitude 14 earthquake that collapses all hydroelectric dams on Earth at the same time. Dozens of cities are wiped out and billions die. All chemical plants are also wrecked and the atmosphere is polluted beyond redemption. All life on Earth ends.
Now, if that isn't terrifying, I don't know what is. And it is equally plausible.