This is probably the source of TSC's story, as part of a report about 'Erratic Boulder Stones at Clun', in 'The Antiquary' for March 1884. I don't know if Coflein have changed their mind, but now they call it a Standing Stone (question mark).
The Beguildy Stone; height above ground, 3ft 6in.; breadth, 4ft. 3in.; thickness - very irregular - from 12in. to 24in., thoroughly rounded at every angle. Many unsuccessful attempts have been made to remove this stone, for standing in the midst of a field, it is an obstruction to agricultural operations. At a depth of 4ft. it is said to spread out to a much greater thickness.
Its parent rock is also in the Rhayader district, though it is commonly believed to have travelled from a different direction; for the popular legend says the devil threw it from the Graig Don rocks, near Knighton, at Beguildy Church; and as a proof the marks of his hand are still pointed out upon it. One of these marks is a bowl-like depression on its upper surface 12in. diameter and 5in. deep.
Craig y Don is a steep hill above the River Teme at SO261737 (the stone is right near the river too).
Another for the list of Radnorshire-disputed-antiquity-stones I fear. From "The Ancient Stones of Wales" by Chris Barber and John Godfrey Williams:
"A standing stone in the centre of a field called Maes y Garreg on Pontycaragh farm (SO200791). It resembles a battered human face and is 5 feet high. It is marked as Standing Stone on Ordnance Survey map of 1947. R.C.A.M. No. 80 of Radnor. One local story is that the Devil threw the stone from his chair at Craig y Don near Knighton, Radnor, aiming it at Beguildy Church, but it fell short by half-a-mile and the stone is still supposed to bear Satan's fingerprints."