Rambling Thoughts:
This whole story felt more like a setup for Alec's eventual confrontations with Mr. Scary and the Midnight Cabal. Everything comes way too quickly to Alec and his Scooby gang.

Just some examples 1. He gets lost in a Shadow Realm he has never heard of. He figures out how to get out right away. While there, he sees Mr. Scary. 2. He looks for The Lady of the Forest's crown and walks into the first bar he sees. He told the local legends there, and of course, it's the place he is looking for. When he arrives, he talks to her sister, and she gives up the crown. No task, no questions, and no hesitation. 3. Felicity's father happens to be researching Egyptian heart curses, and she figures out how to save Mallory. 4. Felicity is tasked to research where the vampires stay in Greece. Now, Greece is 50,942 square miles, and according to Wikipedia, there are approximately 4,394,506 households. Yet Felicity finds the place within minutes. 5. When Alec brings the "remains" of the Lady of the Forest to her sister, The Lady of the Lake, he vows to avenge her death and kill every member of the Cabal. The Lady of the Lake gives him Excalibur. FUCKING EXCALIBUR. He doesn't have to pull it from a stone or do anything heroic. He makes a vow, and she hands it over.

Now I know this is a work of fiction, and not everything will be realistic, but you can make them work for it a little, or maybe not everything goes so smoothly.

Alec and Felicity's relationship has come to a standstill. I believe that's to reignite the love triangle with Mallory (or Deputy Amy) for the future.

Alec is WAY too trusting and honest. He claims to be suspicious of everyone, yet he has no problem telling everyone the truth or his plans within minutes of meeting them. Dude, you can hold some stuff back or maybe lie a little. You're dealing with the Midnight Cabal, and you have no idea who is a member, so perhaps don't tell everyone everything. 

There is finally a sign that Alec is a Harbinger of Death.

After he uses one of his spells, he sees a red circle in his mind. He concentrates on it, and he begins to heal himself. Once he is done, all living creatures around him drop dead

There was the death of someone Alec was supposed to protect. This is the first time he failed, and I am interested to see how he handles it.

There was a pleasant surprise twist, and one of the members of the Cabal was revealed. 

This was the first book in the series with a different character's POV. I get that Felicity is at home in England, but up to now, the series has only been from Alec's POV. I didn't care for it.

I am intrigued by how Alec will handle the Cabal and the big reveal, so I am continuing with the story.