Endless Wrath is a dye for Samurai Clan Armor. It costs 10 flowers at the Crimson Dye Merchant. Like every dye it only changes colours.
The Crimson Dye Merchant is Sao, a vendor added in the Iki Island expansion. He only trades after you beat the Hidden Cove Tournament at Fune's Refuge, and he hands you one item free on your first visit.
Once unlocked it shows up as a colour option for Samurai Clan Armor, so you need that outfit to use it.
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.