Pretty much every interaction Rose and Dimitri have in Blood Promise. It starts off with a mourning Rose hunting her undead lover just weeks after losing her virginity to him. Then, when she finds him, he takes her captive, gets her hooked on his bites, and keeps her as little more than a Sex Slave for months. Even Harsher in Hindsight once you reread and remember how horrified Rose was at the bare inference that she was Lissa's blood whore in book one.
Speaking of Strigoi cruelty, the dhampir squadron getting captured in Frostbite. They were just kids....
Lissa snapping in Shadow Kiss and torturing someone with visions of spiders crawling all over him. Admittedly, he kind of deserved it, but seeing sweet, compassionate Lissa so cold and brutal is unsettling.
