The Iron Seal by J. M. Briggs

Alex and her fellow mages are trying to settle into the routine of another school year, but Arthur and Queen Scathbas are still plotting. As the attacks on their Ravenslake home increase, the mages must discover new ways of using their magic and to protect each other. Questions about the Demons, the Sidhe, and the Tree of Reality linger, and Alex knows she has to find the answers before it’s too late. Dobiemir was a farmer and a father. With his only remaining child sick, that was all he wanted to be. But strange creatures made of cold and shadow are stalking his home. The harvest is dying from their cold, and his son is growing worse. Hope comes in the form of two strangers with great powers who even have the assistance of gods, but they aren’t content to deal with this themselves. They drag Dobiemir into their dangerous world to confront the mad god Chernobog.