According to the National Library of Wales:
He is said to have kept one of his books padlocked and hidden away, and only dared open it once a year in a nearby secluded wood where he would read various incantations from it to summon forth spirits. Once opened, the book was said to create a very severe storm. This led to the notion that the Harries' derived their power from this large thick volume of spells, bound with an iron chain and 3 locks.
This person wrote the grimoire Book of Incantations