Terribly disappointing. This read like Throne of Glass meets Flowers in the Attic in Westeros. Although I like A Song of Ice and Fire and have a fond nostalgia for early VC Andrews novels, this combination is BAD, okay? VERY BAD. 
 The following is laden with spoilers, you have been warned. 
 I didn't like the little I saw of Sybella in Grave Mercy and this book did nothing but increase my dislike of her. She is a mix of a Black Hole Sue and a Jerk Sue with a dash of Sympathetic Sue. Note that I do not think she is a Mary Sue in any way, but she has traits of all three that are extremely obnoxious. Her backstory is piles among piles of bad shit that happened to her because why the fuck not. Her father was abusive. Her oldest brother raped her because sympathetic backstory. She started an incestuous relationship with her other brother and blames him for it. She had a baby and her father killed it. Her step-mother was killed because of her. So she escaped that special level of hell, but the abbess/Reverend Mother throws her back into it because she supposedly hates Sybella. She's a self-proclaimed martyr. She's supposed to be the girl who grew up too fast and is more 'dark and mature', but then we get crap like this: 
 Instead, I cross to the bed, grab a handful of the thick, burgundy damask curtains, wad them up in my fist, then shove the wad into my mouth and scream. The relief of all the anger and fury leaving my body is so sweet that I do it again, and again. 
 Wow, that is SO mature. Kind of like the time that Celaena from Throne of Glass threw a hissy fit and bit into a cue stick because she didn't master the art of billiards in less than an hour. 
 Sybella is supposed to be an assassin. You know, the move-in-the-shadows, garrote you/slit your throat/poison your food types. But no, Sybella spends more time in melee combat against soldiers than silently offing people. That's not an assassin, that's a warrior. There isn't any political intrigue or assassinations being plotted or anything like in Grave Mercy. Nope, Sybella instead follows Beast headfirst into battle. This reads more like an action adventure novel. She trains young boys how to fight when she's supposed to be trained in the art of assassinating, not full blown open warfare combat. 
 The romance between Beast and Sybella isn't believable. She's a bitch to him and he falls in love with her. He's practically dying and she falls for him. It's a dose of instalove that had no rhyme or reason and made me cringe. 
 Her relationship with Julian is messed up. The way she sees it, it's like we're supposed to lay all the blame at his feet, but it seemed to stem from some twisted logic in her brain. She starts sleeping with him because she thought he wouldn't protect her if she didn't. He falls in love with her and she never discourages him. Even when she returns to the D'Albert household, she resumes her relationship with him and silently laments that she hates it and he's manipulative. How? Why the fuck did she never say anything to him? After Pierre raped her, why didn't she tell Julian instead of leading him on? Other than him killing Marthurin, Julian seems like a decent enough person, despite being D'Albret's son. Even at the end, when she tells him their relationship had always been wrong, he seemed to understand and, by her own admission, acted like her brother and not her lover. Sybella is a victim of Pierre's actions, but everything with Julian seems to be of her own making. Pity he had to die. 
 Ismae is the only real bright spot in this book. It just reminds me how much I miss her and would rather be reading about her than Sybella. I even warmed up to her relationship with Gavriel when I was iffy about them in Grave Mercy. 
 We also discover the real meaning of the marques in Dark Triumph. Surprise! It has nothing to do with traitors against the Countess. The marque only appears on people who are about to die. That means that the abbess and the whole order of Mortain claiming the marques only appear on traitors is some political scheme that serves their agenda, whatever that is. I knew the Reverend Mother was shifty! How much still hasn't been revealed, but I know she's up to no good! 
 The reason D'Albret was never marqued was because Mortain promised all his victims that D'Albret would never enter the Underworld. Meaning that he will never fully die and then roam as a spirit forever. So D'Albret could have been killed a long ass time ago and I wouldn't have slogged through this terrible book. 
 I miss Ismae. I hate Sybella. Hopefully Mortal Heart is better.