"Stone activating a creation of Pharros the Vagabond. Pharros the Vagabond was a legend who wandered the lands, creating contraptions to help those in sincere and dire need. The scope of his travels was so wide that the Pharros has been mistakenly credited with many inventions that were crafted by others."
Sold by the Hag in the Forest of Fallen Giants for 4000 souls.
Another is found in the mansion in Majula. Look in the book room and you'll find it on a dead body.
Explorer class starting gear
Sinner's Rise: last flooded cell on the left heading towards boss room.