Tribal Terror is a mask you get by finishing "The Lightning in the Storm", from Tales of Iki. It is a story reward, so playing the tale is the only way to add it.
Play "The Lightning in the Storm" to the end and the mask comes with the rewards.
The Eagle's Chosen
The Eagle's Chosen is a dye for Ronin Attire. It waits in a chest inside Fort Sakai on Iki Island once you finish "The Lightning in the Storm". 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.