Might be The Weirdstone of Brisingamen by Alan Garner.
The book's introduction concerns the origin of the weirdstone. Following the defeat of Nastrond steps had been taken to prepare for his eventual return. This involved bringing together a small band of warriors of pure heart, each with a horse, and gathering them inside the old dwarf caves of Fundindelve, deep inside the hill of Alderley which were sealed by powerful white magic which would both defend Fundindelve from evil as the ages passed and prevent the warriors and their horses from ageing. When the time was ripe and the world once more in mortal peril it was prophesied that this small band of warriors would ride out from the hill, trusting in their purity of heart to defeat Nastrond forever. Fundindelve had a guardian, the ancient wizard Cadellin Silverbrow, and the heart of the white magic was sealed inside a jewel, the Weirdstone of Brisingamen.
At the beginning of the story, however, the Weirdstone has been lost, stolen centuries before by a farmer whose milk-white mare Cadellin had bought to complete the numbers in Fundindelve. The stone became a family heirloom and eventually found its way to Susan's mother, who passed it on to Susan, oblivious of its history and purpose. When the children meet Cadellin the wizard fails to notice the bracelet even when the children come to visit him in Fundindelve. However, its presence does not go unnoticed by Selina Place and the witches of the morthbrood, who send their minions to steal it. Susan finally realises the identity of the Weirdstone and, fearing its destruction, sets out to warn the wizard. The children return to Fundindelve but are waylaid by a dark presence and the Tear is taken. Once they inform Cadellin they are told to keep away, to not further involve themselves. However, whilst exploring on their bikes they notice a mysterious cloud travelling across the landscape before hovering over the home of Selina Place, St Mary's Clyffe and they go to investigate hoping to recover the stone on their own. They are successful but become lost in a labyrinth of mine-shafts and caverns. As the members of the morthbrood and Selina Place, later revealed as The Morrigan, close in on them they are rescued by a pair of dwarves, Fenodyree and Durathror, who are close companions of Cadellin. After passing through many perils the group returns to the farm where Susan and Colin are staying to spend the night, where at midnight The Morrigan menaces them through the door. They set out with the farm's owner the next day to return the Weirdstone to Cadellin before it can fall into the wrong hands. Their travels take them through gardens, lawns, fens, tangled rhododendron thickets, pine plantations, mountain peaks and snowy fields while striving to avoid the attention of the morthbrood.
At the climax of the story a great battle takes place on a hill near Alderley during which the children and their companions make a desperate last stand to protect the Weirdstone. However the enemy forces prove too strong and Durathror is mortally wounded. Grimnir takes the Weirdstone for himself and, in the ensuing chaos, Nastrond sends the great wolf Fenrir (in some editions Managarm) to destroy his enemies. As the remaining companions begin to despair, Cadellin appears and slays Grimnir, whom he reveals to be his own brother and who in the final moment accepts defeat and drops the stone into Cadellin's hand. The Morrigan flees in terror while Cadellin uses the power of the Weirdstone to subdue once again the forces of darkness.
There's a sequel, The Moon of Gomrath.
