Caverns of Fire: Book 2 of The Restoration

Woli has been captured by the northerners, now a slave to one of the female leaders. His twin, Rola, wants to rescue her brother at once, but their masters, Thalamar & Klaybear, counsel that the time is not right. In the south, the search for the missing nefalem leads Telor, with Blakstar and Kovaine, deeper into the nefali mines and an old enemy who wishes to destroy them. What they find shocks even their masters: an old enemy all had thought died during the Great Year.