The Shadows
Volume Three of Zodiac Academy The Reckoning left us with Lionel Acrux feeding the Vega Twins to The Shadows.

At the beginning of the Volume Four The Shadow Princess we learn the sisters have survived.
So has Orion.
So has Darius.
But all of them now have to deal with The Shadows, who have invaded their minds. Orion & Darius are prepared for this as they have been dealing with dark magic for years. And Darcy has Orion to help her. But Tory has a hard time dealing with them. She seeks danger and The Shadows provide that kind of bliss-inducing thrill she craves.
So much the novel is about how our characters handle The Shadows.
Seth blackmails Darcy

Seth learns of the illicit relationship between Darcy and Orion, and uses this info to wield power over Darcy. She is so terrified of Orion being found out that she doesn’t even confide in her twin.
This rift and the misery associated with it plague most of the novel, until Orion mistakes Tory for Darcy and the game is up. To her credit, Tory is very supportive of the couple, who are now very much in love.
Caleb doesn’t see Tory as “the one”
When Caleb admits that he doesn’t see a future in their relationship, Tory cuts him off. So this event, plus her sister not confiding in her leads to her feeling lonely and abandoned during much of the novel. Perfect fodder for the manipulations of The Shadows.
The Voice

One really disturbing element is The Voice that all the characters hear. It is the voice of a young woman pleading to be let free of the shadows. Eventually, Orion works out that this is his sister Clara, whom he presumed to be dead. He makes plans to bring her back.
Eventually, the night comes when Orion plans to get his sister back. The plan is for Tory and Darius to come and participate, to lend their power. But they never show. Instead they get caught up in some weird kind of magic that leads to their being “Star-Crossed” lovers. In the meantime, Orion with Darcy’s help gets his sister back. But he pays the ultimate price.
The novel ends with a distraught Darcy screaming for help.