system
Given the following post, respond as if you're commenting on Reddit:
human
[Dune] Please tell me more about the universe in general, anything really.

Things I've got down:

* Takes place in the far, far future, like 20000 years
* The novels are really detailed
* There is an Emperor, but I don't know a thing about him
* Spice is a hell of a drug
* Desert world with sand worms that eat people
* Robots are banned
* Warhammer 40000 borrows from it heavily

That is all I know. Just tell me anything I *should* know about dune.
gpt
Dune is set around 20,000 years in the future, long after a lot of "regular" scifi tropes have come and gone. Humanity used to use sentient machines for a lot of its heavy lifting, but as usual, they rebelled. Unlike many other scifi universes however, the war between man and machine was SO brutal that not only was artificial intelligence outlawed, it was made religiously heretical. 

Humans have kept many ancient traditions, languages and religions from old Earth in this distant future. In fact, most religions have come together into beautiful new combinations. Most people follow the Orange Catholic Bible, which combines all major Abrahamic faiths and some others. Naturally, one of the most important tenets of this religion is the hatred of AI. 

Humanity used to use standard FTL and cryo-sleep ships to travel between the stars, but the Spice changed all that. People who are infused with enough of it (which is an extreme amount) become navigators, who have the ability to fold space and make ships travel instantly. The spice also extends human life by hundreds of years and expands human mental capacity, allowing specially trained people to do intense mental calculations just like computers of old used to. They're called mentats. 

Politically, this spice-dependent empire is a tripod, the most unstable political arrangement. There's the Emperor at the top, the great houses and the spacing guild, which holds a monopoly on ALL legal space travel in the empire. The houses control individual planets or small numbers of worlds and do much of the day-to-day ruling. A single powerful great house is usually responsible for mining all spice on Arrakis and legally selling it to everyone. House Harkonnen has ruled Arrakis for a long time by the beginning of the first novel and are ludicrously wealthy. 

Arrakis is 100% a desert. There used to be abundant life and water on its surface, but the sandworms changed it over time. People do not wish to change this because the sandworms also make the spice as a by-product of their life cycle. It's complicated, so just remember that the huge and deadly sandworms are essential for the spice and the spice is essential to the worms. A group of reclusive people live independently on the surface. Called the Fremen, they live outside the Empire's laws but usually don't wish to change the status quo... until their savior arrives.