Story/novella about a giant hand from another dimension/universe

I read a story in the sixties, perhaps written earlier, about a post medieval but pre-industrial world where a common fellow gains the ability to call forth a giant hand that he manipulate to do his will. The hand, fist as big as a VW bug, manifests from thin air to battle enemies, destroy and build and is a creation of non magical means. Transdimentional, metaphysical, cerebral mutation,... not sure what the explanation was. Think Green Lantern without a ring and only one trick up his sleeve. It didn't hang around, floating along with the main character, it would appear when needed. And I'm not sure if this was a manifestation of his own physical self or an other world entity. Needless to say it can't be kept secret and the events snowball ala Game of Thrones.

Sorry that I can't recall author or title and if I could, why ask this question. Thanks for any help. My Cthulu Mythos story search never got an answer, still... hoping.