Mask of the Eagle is a mask you get by finishing "The Blessing of Death", from Tales of Iki. It is a story reward, so playing the tale is the only way to add it.
Play "The Blessing of Death" to the end and the mask comes with the rewards.
Unburdened Heart
Unburdened Heart is a dye for Samurai Clan Armor. It is a reward for completing "The Blessing of Death" (Tales of Iki). Like every dye it only changes colours.
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.