Rogue Mage book 3: Host
Faith Hunter2007

In a post-apocalyptic ice age envisioned by Faith Hunter in Bloodring and Seraphs, humans continue their fight to survive amid the warlike forces of Light and Darkness—and struggle to understand the seraphic and demonic beings who have invaded their world....

After risking her life to save the residents of Mineral City from a fallen seraph of the underworld, Thorn St. Croix finds herself finally accepted, albeit warily. Not so the mage who arrives from the Council of Seraphs, and who could be her greatest ally—or her most dangerous foe.

But a terrible danger threatens Mineral City. The bonds of the Dragon have been loosed and it will soon be free. Darkness and its minions overtake the town, setting off avalanches and cutting off communications. At the mercy of Darkness, Thorn must rally the townspeople together—for only as one can they survive. But her loyalty will be tested by someone she once lost to the Dark: her twin sister....