Made famous by Dan Brown's book "The Da Vinci Code". The orginization was created as a fraud in the late 1960s, though it continues to exist as a myth.
"Did you know Isaac Newton led the Priory of Sion"
"Dude, that ain't real"
The Priory of Sion was/is a brotherhood that keeps/kept an ancient secret concerning the heritage of Jesus Christ and The Holy Grail
. They established the Knights Templar and cherged them with excavating the site under Herod's Temple in search of the grail itself.
He was the leader of the Priory of Sion.