Vengeance - Douglas Hill

After a violent confrontation with Fray and Kannis, two of the Cartel's more senior mages, Nick and Julia, lie wounded, assumed dead, while Paddy, April and Sam have been captured by demon forces.

But Nick and Julia aren't dead. With the help of the learned Alderon and the creepy Bertrand Nowell they race against time to find their friends before the Cartel destroy their minds and keep them enslaved, doing their evil bidding, forever. Armed only with his trusty knife and his incredible quick-wittedness, this is Nick's toughest challenge yet - to enter the heart of the Cartel's domain and destroy it or . . . die trying . . .