I somehow didn't catch the anti-magic fields in the update, and in your description I thought it was just "being opposed to the use of magic".
Ah. No, they have anti-magic auras. A fairly logical thing to give your witch-hunters, and it has been done in various other pieces of fiction. It was just the anti-magic
and religious language/name that lead me to the Templars.
I'd estimate about half of the worlds being deathworlds. Or all of them being half-deathworlds.
I'd say they're all at least a little death-world. The level to which the population has adapted to the danger may vary wildly, though. Still, unless the danger is new, if there is a population, chances are they've adapted at least a little.
There are some antimagic people. Religious nuts like to weaponize them.
Now that you mention it...yeah, I vaguely remember something about like...rank Omega psykers?
Interesting that the Houses here are extreme traditionalists. I'm guessing part of a mageocracy of some sort before the Inquisition, or perhaps in the distant past before a scientific revolution (they produce complex fireworks) or industrialization. Why do I say this? Because their (apparent, very biased) reaction to an antimagic aura is to try to use a spell through it. You know, instead of a gun or a crossbow or using magic to hurl a boulder.
Eh. Given that the Inquisition also lacks fire-arms (they use knives instead, apparently), that might be for a reason.
The Inquisition also likely has more than one trick. But I would say they're also more of a symptom than a cause. If, lots of if speculation in this post, If the Houses are strongly traditionalist and positioned themselves as nobility for thousands of years, then the current state of affairs changed massively before the Inquisition came along in the last few decades. Some big change in the structure of society happened, maybe even a while ago, so that Norms having an opinion actually counts for something now.
At a guess, I'd say many of the Houses may have buggered off to other worlds, reducing the remaining House's numbers down to a level where they couldn't maintain as much power. They may have subsequently gone underground, as they weren't public enough figures for the Inquisition to find all of them.
Of course, they might always have ruled the world from the shadows, and the Inquisition brought their actions into the light, perhaps breaking their secret control over the world in the process. Maybe that's why the Inquistion is so common and accepted. There are a lot of possibilities here, so we should probably try and narrow it down.