Is about a trio of like 'judges' that travel around the galaxy and 'exterminate' civilizations that have become too evil.
And basically there is a humanoid species that had developed a horrifying type of 'communism', and those 'judges' destroy the planet.
Volcanoes and slavery are mentioned.
No further details because of spoilers and it should already be easily recognizable.
If anyone here has read that short story, or knows the title or author, then please do tell.
There are plenty of things to say about it; but in hindsight what made it stick with us is the way its various themes align with experiences of plurality / dissociation.
As inside, so outside, and vice versa. Structures of relationships, and of societies, are often replicated within the minds of their inhabitants.
I did not realize almost four years had passed since reading that.