A horse turns into a nightmare. A cow turns into an evil cow. Characters who aren't evil can lose production on hell terrain. I'm not sure on all the effects. Civopedia should have entries for the effects of hell terrain types. If not, you can learn about them through the world builder.
I haven't had that random event. Most of them don't do you any real harm. They just add a bit of randomness to the game.
I haven't had that random event. Most of them don't do you any real harm. They just add a bit of randomness to the game.