Archaeologists and volunteers have discovered a stone bearing a mysterious inscription and carved birds that the Picts of Scotland crafted more than a millennium ago. The cross slab, found in a small cemetery last month, dates to between A.D. 500 and 700, and sheds new light on the historic interaction between heritage and faith in the northern U.K. The Picts, or “Painted People,” were so-named by Roman historians because of their supposed war paint and tattoos (“picti,” is the Latin word for “paint”). They lived in northern and eastern Scotland in the early medieval period. Likely descended from Celtic tribes,...