Druids

Rome burned their groves and kept no notes. What they knew went underground.

The priestly class of Iron Age Celtic societies. Nearly everything known comes from hostile Roman sources — Caesar's claim of human sacrifice and wicker men, Pliny's mistletoe rite — plus archaeology like the Lindow Man bog body. They left no writings; Rome suppressed them (Anglesey massacre, AD 60). The vacuum was filled from the 1700s by Romantic revivalists who invented robes, Stonehenge ceremonies (the stones predate Druids by two millennia), and unbroken lineages. A founding case of a secret priesthood reconstructed almost entirely from its enemies' reports.