To Genzo in Komatsu Forge is a written record from Ghost of Tsushima's Records set, left by locals during the Mongol invasion. You find it at Komatsu Forge in Izuhara, and it counts toward the Avid Reader trophy.
Go to Komatsu Forge in Izuhara and look for the paper or scroll to read. Komatsu Forge is a Mongol-held territory, so expect to fight or sneak past guards before you can search it.
Also at Komatsu Forge: the record The Black Sands and a Hidden Altar in the cemetery.
Komatsu Bamboo Strike
The Komatsu Bamboo Strike is a sword-timing challenge near Komatsu Forge in Izuhara. Cut through all the bamboo with the right buttons and it counts toward raising your maximum Resolve.
The Black Sands
The Black Sands is a written record from Ghost of Tsushima's Records set, left by locals during the Mongol invasion. You find it at Komatsu Forge in Izuhara, and it counts toward the Avid Reader trophy.
Aaruul
Aaruul is one of the 50 Mongol Artifacts, pieces of the invaders' culture and warfare you can study. It sits at a camp south of Izuhara Clearing in Izuhara, and finding 20 artifacts counts toward the Know Your Enemy trophy.
Adachi Dragon Mask
Adachi Dragon Mask is a New Game+ mask sold by Baku the Voiceless for 5 Ghost flowers. It does not exist in a first playthrough.