Before the Storm spoilers below
I've read quite a few Warcraft and World of Warcraft books, but none of them come close to Before the Storm in terms of how dark it is. The whole thing about humans reuniting with their Forsaken relatives, only to see them being slaughtered in front of their eyes. All on Sylvanas' orders, 'cuz she couldn't deal with the fact that some Forsaken had stepped up to govern the Undercity in her absence.
God, just fuck that woman. Why in the world do the Forsaken and the Horde follow a bitch who rains down arrows on her own people? This book makes it clear there's no redemption to be had for her, I just hope she finally dies real soon. And that we get to kill her.