These are what we call the 'YMMV items.' Things that some people find in this work. We call them 'your mileage might vary' because not everyone sees these things in the same way. This starts discussions in the trope lists, a thing we don't want. Please use the discussion page if you'd like to discuss any of these items.
Nausea Fuel: Some of the Gorn, and a few demon realms as well. Like that one from The Board in Demon Thief. You know the one...
Nightmare Fuel: Begins just from looking at the cover of the first book, and once you actually read it...
What stands out particularly is the ending to the book Bec, where the titular protagonist/narrator gives a rather detailed description of the various demonic beings she is surrounded by while trapped in a tunnel underground, as the light fades into darkness. And then gives a very graphic description of the sensation of being eaten alive by the various creatures (with claws, teeth, beaks and tentacles), with the implication that the last thing she ever hears is the sound of her own screams in the dark.
In another incident, a guard fires two bullets at a demon lady, who proceeds to stop both bullets in mid-air, transform them into demonic butterflies, and return to sender. More specifically, his eyes, and from there, his brain. Slowly.
Slowly Slipping Into Evil: Grubbs does this toward the end of the series after he allows his Wolf side to surface and, in his words, "becomes a mix of a human, magician, wolf and Kah-Gash." These acts range from allowing Antoine Horwitzer to be slaughtered to ripping out Kernel's eyes because he refused to help with Grubbs's cause. He gets better though.
Bec confronting Dervish about simply using her to relive every moment of Bill-E's life.
What Do You Mean, It's for Kids?: The Demonata is quite possibly the most extraordinarily hyperviolent children's series ever written. One of the tamest examples is Grubbs throwing rats guts on his sister while she showers, just a few pages before he comes across her and their parents savagely murdered corpses, in a room covered with their blood. Decapitation, dismemberment, infanticide, impalement, torture, mutilation and all manner of horrible deaths occur in every book, sometimes dozens in a single scene. To say nothing of all the bullying, kidnapping, animal cruelty and all manner of sins that the heroes get up to.