Halt's Peril by John Flanagan

The renegade outlaw group known as the Outsiders has journeyed from kingdom to kingdom, conning the innocent out of their few valuables. Will and Halt, his mentor, are ambushed by the cult's deadly assassins when Halt is pierced by a poisoned arrow. Now Will must travel day and night in search of the one person with the power to cure Malkallam the Sorcerer.