Demon Sphere is a planet roughly the size of Pluto, which is absolutely covered in cityscape. It is continually night, and the architecture resembles gothic architecture, so lots of spines and the occasional splash of colour via stained glass windows. Most of the residential buildings, the only exception being the Houses, are skyscrapers which function more as hives than anything resembling a home.
The dominant race of Demon Sphere are Tieflings, which is quite a feat considering that in this universe they’re infertile. The seven Houses of Demon Sphere breed with humans to essentially sire their entire army and community, and each tiefling owes its allegiance to one of the Houses (in theory, anyway). At the very least, each tiefling sired by a Lord bears some sort of physical markings that indicate they were made by that House. The colloquial term for this effect is the blood curse.
As a result, humans who are generally left to their own devices in the city have formed into one tight-knit area known as “human territory”. Human society is hard to get into, but easy enough to fall out of. Falling out is usually associating with tieflings (or “demons” as everyone generally calls them, even though it’s a misnomer) in any fashion (working for demons, being friends with demons, having consensual sex with a demon, submitting to the rule of one of the Houses, being pregnant with a demon whether or not the sex was consensual), and since leaders of rebellion run the place, they can out whoever they want. If made clear that a person really genuinely regrets their actions, the leaders may be forgiving. Or they might turn you out into the cold.
The Houses would march on the territory if humans didn’t naturally breed themselves faster outside of captivity, and letting them have the hope of seeing a new era of change is enough to sustain them. Thus any tiefling who wanders into human territory is written off as a loss, and everyone moves on.
So, to summarize, Demon Sphere is a horrible place to live.
Each House is run by a Demon Lord. Because of the nature of Demon Sphere, it’s only full blood demons who manage to claw their way to the top. Running a House is a difficult job, and generally the first thing that qualifies someone to do so is fertility. The forces sent out in the name of any one House are directly descended from the Demon Lord who runs the show, even with Houses run by females. After that small hiccup, it’s just the case of managing martial, diplomatic, and personal control over the House, and between the other Houses until successfully established. Naturally, no one has managed it since the formation of the Sphere.
There are 7 Houses in total.