If an asteroid smashed the Earth, humanity would be beaten down, but it would likely survive.
If aliens invaded the planet, humanity would suffer, but it would fight to the last man.
What if both happened?
A massive starship appeared from nowhere in our upper skies, perhaps a miscalculated warp trajectory, and crashed to Earth. Fragments the size of cities rained on various parts of the planet. Billions died from the catastrophe.
Any attempts to investigate impact sites were met by automated defense systems. Humanity suffered even more with no answers. Then we thought we could recover until things emerged from the wreckage. Horrible nightmares and beings with incredible power.
Suffice to say the Earth has changed. Its environment is ripped and seared with radiation. Population replacement is at abysmal levels. Those that do survive are being enslaved or struggle for survival.
Someone has risen from the pits. Someone has finally given us hope. If anyone would be named a hero these times, we would call Thundarr the Barbarian one of them.
And so we begin the chronicles of his work and travels in the deadly days of Earths end. Or Earthend.