From “Mysterious Wales” – Chris Barber (Paladin 1983):
This stone, 7 feet high and 5 feet broad, stands in the middle of a field to the west of Llanfihangel Rogiet church. One historian suggests that it was placed in the field to mark the height to which the water rose on the occasion of the Severn flood in 1606. The legendary origin is much more interesting. It was hurled from Portishead, or some other spot on the far side of the Bristol Channel, by the Devil in a fit of temper!
Sadly Barber doesn’t give any source for this legend.