Matthew Ward 
Frostgrave: Second Chances [PDF ebook] 
A Tale of the Frozen City

Time is running out for Yelen and Mirika Semova. Though the sisters have earned an enviable reputation amongst their fellow explorers of the Frozen City, their lives are haunted by a curse the more Yelen uses her magic, the closer the demon Azzanar comes to claiming her, body and soul. But Azzanar is not the only one manipulating Yelen and Mirika…When catastrophe separates the Semova sisters, it falls to Yelen to save them both. But in a city shrouded in deceit, who can she turn to for help… and what price will she pay to get it?