Complete Monster: Gerridon, in the distant backstory, was the greatest leader of the people, Highlord of the Kencyrath and Master of Knorth, but he would go down in history as their greatest traitor. Desiring immortality—in a Martyrdom Culture—and bitter that the absent Three-Faced God wouldn't grant it to him, he sold the souls of his race to their ancient enemy, Perimal Darkling, in exchange for eternal life and manipulated his twin sister Jamethiel Dream-Weaver into reaping them for him. This betrayal would not only destroy two-thirds of the Kencyrath, but leave scars in their collective psyche that would linger for millennia. When Gerridon's immortality began to fail him he sought out various means of sustaining it, including attempting to create a child of his or Jamethiel's blood to take over as Dream-Weaver—by rape if necessary—and taking command of an ancient religious sect and whipping them into a fanatical fervor in which they brutally tortured hundreds of Kencyr prisoners of war, including Gerridon's own nephew, in an attempt to force them to convert. Driven solely by his selfish desire for immortality, Gerridon will sacrifice anyone and anything—including his own blood-kin—to save himself and prolong his life. Not not only did he betray his people and his god, but he plans to do the same to his "master" Perimal Darkling, if he can find a way to maintain his immortality outside of its control; his pride rankles at the thought of being subordinate to anyone or anything. As Jame observes, there is nothing Gerridon won't do to keep on living and to avoid paying the karmic price for his long-ago betrayal, and his remains a unique and infamous position in the long annals of the Kencyrath's history.
Rawneth is described as darkly seductive in the very best way, especially in the short story and flashbacks that take place in Ganth's youth.
Also Kallystine, in very different way. While Rawneth is sexy in a Femme Fatale way that knowingly exploitsEvil Is Sexy, Kallystine doesn't, and so she comes off very differently—like something overly-sweet with a bad aftertaste.
Launcher of a Thousand Ships: Kirien is awesome and likable in a kind of general, adaptable way. Kirien/Jame? Plausible. Kirien/Tori? Plausible (and pointed out by Trishien). Kirien/Kindrie? Plausible (and eventually canon).
Moral Event Horizon: How Gerridon became immortal in the backstory. Jamethiel Dream-Weaver may have committed the actual soul-reaping, but it was Gerridon's idea and he was the one who benefited, so it counts as his crossing more so than hers.
Nightmare Fuel: The description of day-to-day life in Ganth's place of exile (mostly in the short stories) where the vegetables were animate and there was so little meat that one of the guards resorted to cutting off and eating his own toes. And the way Ganth's horse died and became a haunt—and wasn't all that different personality-wise? Perhaps the squickiest part was not so much the description of the things themselves, but the way Jame and Tori and all the others accepted them as a part of their "normal" life.
Situational Sexuality: In the Women's World. Whether or not sister-kinship would exist if Kencyr society didn't work so hard to separate and/or make enemies of the sexes is unknown to all but the author.
Squick: As much horror imagery as there is in the series, there is bound to be something that will squick you. (See Nightmare Fuel above.)
It is also possible that some might be squicked by the relative commonness and casual acceptance of incest, especially among the Highborn families not to mention the increasing hints that Twincest is in Jame and Tori's future. It's one thing when your villains are incestuous, but quite another when it seems predestined for your hero and heroine!
When Gorbel's foot is infected with the willow. The idea of that, and the imagery, just really freaked me out.