This book isn’t exactly bad, and I fear the German translation also didn’t do it much justice. In German, everything sounded kinda… off. There were weird phrases used and wrong words chosen, while the one chapter I read in English  sounded way more coherent and typical of a fantasy book. 
Apart from the writing, the characters just weren’t interesting (we have the „rough, traumatized (but not really?) and emotionally cold protagonist, wo doesn’t want to let people close to her lest they get hurt. Then there’s the good-hearted younger „friend“ who has absolutely no character or flaws and is only there to make the protagonist softer and also the father figure for both of them, with little to no explanation why he even chose to help those two strays. Last but not least there’s a mysterious but intelligent love interest/possible friend turned sister (yippee ;-;) who has to teach the protagonist all about magic in the span of 3 weeks. 
The plot wasn’t bad per se, but - once again - it could have used some more explanation and thought behind it. 

Yeah, not really my cup of tea.. but hey, it wasn’t terrible? 

Honestly, I think I liked this one best out of all the volumes so far. Especially Nicos (small but really impactful) storyline gave it a bit more giddiness. And of course, who doesn’t like gruesome descriptions of Tartarus?