Frost Bite by Richelle Mead is book two in the Vampire Academy series. I finished it a while ago so this will be brief!

Strigoi are working with humans to attack royal families. The Moroi are fearful, but Christian’s aunt Tasha advocates them to use their powers to fight alongside their guardians.

Lisa and Christian develop a relationship while Rose is in a love triangle between Mason and Dimitri. The story reaches a climax when Rose, Christian, Mason, Mia and Eddie are taken prisoner by Strigoi. Not everyone will survive.