The Weirdstone of Brisingamen

Exploring Alderley Edge in Cheshire, Colin and Susan are chased and captured by sinister creatures. Cadellin Silverbrow rescues them and takes them to Fundindelve, deep inside the Edge, where he watches over one hundred and forty knights who lie in enchanted sleep until the day 'when England shall be in direst peril and they must ride and drive the enemy into the sea.'
Fundindelve was guarded by the strongest magic the world has ever known, magic that would keep the warriors from growing old and weak. The heart of the magic was sealed within Firefrost, the Weirdstone of Brisingamen.
But the wizard Cadellin has lost the Weirdstone of Brisingamen, and the evil forces of Nastrond are searching to destroy the stone and reign over the world.
Colin and Susan must risk their lives in the quest to return Firefrost to its rightful guardian.