Seekers of the Fox (Thieves of Shadow #2) by Kevin Sands
A dying friend, a terrible bargain, an underwater quest, and dangerous magic . . . this adventure picks up right where Children of the Fox leaves off.
Rule number one: Never mess with magic. Even so, a life-or-death situation calls for Callan and his criminal friends to make a deal with the Eye—the sinister, sentient artifact they stole from a sorcerer. Something precious in exchange for a future task, and the gang has no choice but to agree. The cure—and their part of the deal—lies with the Dragon's Teeth, a pair of swords with extraordinary powers, and the search for them leads the thieves on a quest that will unravel the mystery of the Eye.-Goodreads