Michael's Crag

Michael Trevennack is an elderly English civil servant who spends his holidays on the Cornish coast staring out at a rock formation known as St. Michael's Crag. Fifteen years earlier, a hundred or so feet below, he and his only son were stuck by falling rocks. The boy was killed, while Trevennack was left with a blood clot in the brain that has him convinced he is the archangel Michael. (Goodreads)