Helm of the Lower World is a helmet hidden on Iki Island. It sits in a secret storage room on the Mongol warship you dock during "Jin from Yarikawa", at Fune's Refuge.
Board the Mongol warship from "Jin from Yarikawa" and search below deck for the hidden storage compartment. The helmet is in a chest there.
The Hidden Heart
The Hidden Heart is a Sword Kit from Iki Island. You win it by beating the Hidden Cove Tournament, the bokken contest at Fune's Refuge, and it changes your blades' look only.
To the Raiders of Iki Island
To the Raiders of Iki Island is a written record from the Records of Iki set, left by locals during the Mongol invasion. You find it at inside the Mongol ship at Fune's Refuge on Iki Island, and it counts toward the Avid Reader trophy.
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.