It's a bit of local legend. Outside the wall of the cathedral itself, there is an epitaph laid into the ground, it belongs to a woman called Mary. It is said that the Devil himself leapt from the top of the tower on the cathedral, and one of his cloven hooves landed half on her tombstone and half in the grass - where the front of the hoof hit the stone, you can see the cloven grooves, and where the back part lays on the ground, no grass grows there (and all my life I have never seen any grass grow there) it is huge, too, I can fit both my feet into each part of the cloven imprint.
Did you ever get to go to Nevern church? There we have the legend of the bleeding trees, if you know where to look, you can see the bloody fluid oozing from a severed tree limb.
Really sorry for the late reply! To be honest, I didn't stray very far from the interior of the ruins (I have arthritis and was pretty achy that day) - now you're telling me all this I wish I'd pushed myself a little more! I love stuff like....cloven hooves and bleeding trees, what's not to like - sounds like a perfect excuse to go back!
I hope you manage to visit again soon! Where I live has so many strange legends and history that's known to locals but not really written about in tourist guides etc. I suggest doing some internet/library digging before your next visit! Then you can visit all our ancient burial grounds, stone circles and all the other strange things we have around here