The Schoolboy who met a Supernatural Entity

An encounter I had as a ten year old schoolboy with something supernatural. A Ghost? An Entity? I do not know. Even then, however, I instinctively knew it was of another dimension, from another realm of the unknown.
Nonetheless, a true tale. One I have never told anyone since 1972, with the exception of my Mother who served my dinner shortly after the encounter, folded her arms and told me she had heard monks had lived on that spot hundreds of years previously, to be close to the priory over looking the river mouth.
She believed me. And that was enough. She had a Mother's instinct for distinguishing tall tales and the truth. The field where this happened has long since been built over with housing. The tree, however, is still there! Despite my inquiries, no resident has seen this Entity since. So far.