Demonic Archer's Mask is a mask lying in a hidden room behind a waterfall at Lone Spirit Falls, Iki Island. It has no map icon, so set the Guiding Wind to vanity gear to find it.
Look for the Grapple Hook point by the falls; the room is beside it. A Sly Cooper easter egg is painted on the cave wall next to the mask, and the text links it to a mad archer obsessed with Uchitsune.
A Scrawled Message
A Scrawled Message is a written record from the Records of Iki set, left by locals during the Mongol invasion. You find it at a camp south of Lake Nagata on Iki Island, and it counts toward the Avid Reader trophy.
Attached to an Old Crate
Attached to an Old Crate is a written record from the Records of Iki set, left by locals during the Mongol invasion. You find it at a shed northwest of Lone Spirit Fall on Iki Island, and it counts toward the Avid Reader trophy.
Image: Sucker Punch Productions / PlayStationBamboo Strikes
Bamboo Strikes are button-timing challenges where Jin slices a row of bamboo poles. There are 20 in all, 16 on Tsushima and 4 on Iki, and completing them raises your maximum Resolve.
Beware the Tengu-Masked Archer
Beware the Tengu-Masked Archer is a written record from the Records of Iki set, left by locals during the Mongol invasion. You find it at a house in Yahata Forest on Iki Island, and it counts toward the Avid Reader trophy.