Undying Rage is a Sword Kit you buy for 15 flowers from the Crimson Dye Merchant. It changes the look of Jin's katana and tanto and gives no combat bonus.
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.
Its colour comes from the dyes of Sao, the Crimson Fisherman, who runs the Crimson Dye Merchant stall.
Every merchant Sword Kit costs the same 15 flowers, which you pick up from the wild plants across Tsushima and Iki.
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.