YA fantasy, magic/ability based on eye color, main boy and girl character, mentor of the two is killed

[Kassandra]

I read this book maybe early this year or last year, and I assume it was published some time in the 21st century. The main 2 characters have the eye color that allows them to see portals into the land of the dead. The king and possibly the royal family are dead people that have been retrieved from the land of the dead by people with the portal eye color. The catch is that anyone brought back from the underworld can't be looked at or something or else they'll turn into bloodthirsty monsters. They also get a little volatile after being re-alive for a while, which is why the king has to be killed every once in a while. Also, to bring the king back from the dead a member of the royal family has to be brought to the underworld too and/or sacrificed. The person in question for this resurrection is a girl who the main girl likes and might be interested in science. In the underworld space is weird and it's very hard to navigate and I believe milk and honey are important sustenance there.

The mentor of the boy and girl has been killed in the underworld and they go to investigate. I quit part of the way through the book but at the point I was leaving there was some kind of fight in the middle of a party with nobles in attendance and there was something about the girl being burnt.

[Rainbowheart]

Did you read it as a physical book or e-book?

Were the main characters teenagers or younger children?

[Kassandra]

I read it as an e-book and the characters were maybe 16+.