The word “Nixian” turns up plenty on a google search, but to my knowledge has not been used to describe Garth Nix in the same way as, say, Lovecraftian or Moorcockian or Dickensian. Of course for the -ian suffix to work you need to either

• have an uncommon name, or
• have gotten there first.

If you’ve got a bit of a boring name then there’s less chance of it becoming the one-word definition of your subgenre, no matter how interesting your writing is. And as for the second, the fact is a lot of the people who got there first are male and white (q.v. my three examples above). LeGuinian is definitely a thing (see Leguinian Jump in the SF Dictionary of New Criticism), and I kind of feel Butlerian should be wrestled away from Frank Herbert on principle. But that’s for another time…

Anyway, “Nixian”. Using it because it’s useful, because I feel it has more depth than just being yet another fantasy setting. Consider this:

In the Nixian genre, Death is about fear and about regret and clinging to life and being unable to move on. Everyone has to go through Death to move on, regardless of how good or evil they were in Life. And there are terrible things that lurk in Death, the Greater Dead which may waylay and subjugate souls and absorb or control them. Death isn’t restful, it’s the start of another fight. It’s about being caught between the horror of what comes next and the horror of eternal slavery to the Greater Dead. There is no benificent lord or master in Death. You die, you’re pretty much on your own, and if you can’t get through the Ninth Gate in time you’re subject to the whims of necromancers and the Abhorsen and Fifth Gate Resters.

Also in the Nixian genre there are a great many beings branded as “Free Magic” creatures who the Charter claim are inimical to human life. Many were subjugated by humans in the forming of the Charter, and bound into things or locked in glass cases or forced into bottles wrapped with golden wire. The humans insist it’s because those Free Magic creatures were wild and dangerous and inimical to life that they had to be locked up for good. Yeah, right.

So the Nixian genre is all about humans who serve the Charter and its fragile hierarchy, and are therefore afraid of everything the Charter says is bad, including dying, Death, the Free Magic creatures they don’t understand, and other Humans who reject the Charter as a Bad Thing.

None of this should be a surprise — as everyone knows, the Shadows were the good guys all along.