The Fool's Phylactery was a hoax perpetrated by a charlatan in the 1600's. The charlatan crafted the container, forged some scrolls, and sold it to a peasant. This peasant attempted to capture the Darkness with it, only to get eaten instead.
Johnny Powell asserts that there is no such thing as a phylactery in black magic. He adds that the relic got its name because countless people died trying to capture the Darkness with the "phylactery" for the past 400 years.