In the minds of your enemies
There is no actual “worst thing you could possibly do” to your enemies; even Dante had trouble figuring out how to really torture the damned, and he had all the faculties of both poetry and his idea of Hell to help him.
But if I truly had to wish Hell on anyone, it would look like this:
1. They would decide I had done something horrible to them,
2. something I have never done to them,
3. and have all their friends reinforce it and say, “Whatever it is, I believe you!”,
4. and have their friends start thinking, “and if he did THAT horrible thing to THAT horrible person, I wonder what horrible things happened to me,”
5. “…or MIGHT have happened to me, such that, if I was even in the same building as he was, I was probably in horrifying danger,”
6. and have THEIR friends say, “as long as I exist in the same UNIVERSE as that Monster, horrible things might happen to me,”
7. so the original person has tons of reinforcement of people who say, “the worst things have happened to you!”,
8. so that person believes, “The worst things have happened to me!”,
9. and that becomes the story in their head, forever,
10. and I, I don’t even know that they think this thing happened,
11. so they can never resolve it, and
12. they block me to make sure I never know what it is they think I did and,
13. their pain goes on forever.