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.
Play "The Blessing of Death" to the end; the dye comes with its rewards. It cannot be bought.
Once unlocked it shows up as a colour option for Samurai Clan Armor, so you need that outfit to use it.
Mask of the Eagle
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.
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.