The most obvious is the 1986 Chernobyl event for me.
A more severe one? Alright, it’s your scenario - I just listed some possible divergence points.
I think the worst case scenario for Chernobyl was that Eastern Europe would become uninhabitable. I’m not changing the Chernobyl exclusion zone’s size though, instead the Chernobyl disaster will be caused by alien infiltrators for the purpose of conjuring a colossal alien demon thing. In my story the USSR is basically the last country on earth that’s not secretly controlled by aliens.
Ah, what nerds on the old alternatehistory.com forum used to refer to as ASB (Alien Space-Bats), i.e. something so improbable that it required the intervention of something like the creatures mentioned before.
