Book or series about stones and a magician named Gwydion

Question

I am looking for a book or series that I read about 15 years ago. I can't remember all that much about it:

The man character was a boy or young man, picked up by a magician named Gwydion, and becoming an apprentice.
The overarching storyline had something to do with some kind of stones of power (think size stones to boulders) buried in the ground, forming a kind of network. This network had been tainted somehow.

There was an order of monks, known for their red hands from excessive handwashing.

I think I also remember an antagonist being freed, after being sealed inside a tree.

Answer 1

This is The Language of Stones (1999) by Robert Carter.

The Realm is an England that is still-magical. Legendary beasts still populate its by-ways. It is a land criss-crossed by lines of power upon which standing stones have been set as a secret protection against invasion. But the power of the array was broken by the Slavers who laid straight roads across the land and built walled cities of shattered stone.

A thousand years have passed since then, and those roads and walls have fallen into decay. The dangerous stones are awakening, and their unruly influence is calling men to battle. Unless Gwydion and Will can unearth them, the Realm will be plunged into a disastrous civil war. But there are many enemies ranged against them: men, monsters and a sorcerer who is as powerful as Gwydion himself.

_{^{Fantasy Book Review - The Language of Stones by Robert Carter}}