Master of Fiends — Douglas Hill

In a barbaric, demon-infested world, no one stands against the all-powerful magic of the Unnamed Enemy, who is called Master of Fiends. No on except young Jarrel and his three friends - Scythe, Archer and Mandra - each with their own special powers. And they have taken on the impossible task of rescuing their friend, the wizard Cryl, who is held captive beyond the Barrier Peaks.

Trying to pass beneath the Peaks, they must battle the weird beings of that underworld and their monstrous ruler, the Keeper of the Chasms. And beyond those terrors lies something even worse - the ghastly realm of the Enemy, and his demon horde from the Farther Darkness...