The Terror of Otsuna is Ishikawa Tale 7 of 9, the last of his Act 2 tales, where you hit Tomoe's Otsuna training camp and finally see her face to face. It starts west of Old Toyotama Hills and rewards the Charm of Efficiency and 2 Silk.
Part 7 of 9 in Ishikawa's Tales
Part 25 of 66 in Tales of Tsushima
Find Ishikawa inside a farmhouse and go with him to talk to a woman named Ran. Walk with him, then travel together to Tomoe's training camp. Fight through it, including a big cat, and search for signs of her. This time she is close by. Chase her down until you reach the spot where she fell, then go back to Ishikawa.
Find Ishikawa in the farmhouse. Go with him to talk to Ran, then walk together. Ride to Tomoe's camp. Kill the big cat and search the camp. Chase Tomoe. Look where she fell, then go back to Ishikawa.
Tomoe taunts Jin into following her to the edge of a cliff, then jumps into the water below. When Jin looks down there is no sign of her. She is not dead, and she shows up again in Act 3.
The Sensei and the Student
The Sensei and the Student is Ishikawa Tale 2 of 9, an Act 1 side tale in Hiyoshi. You and Sensei Ishikawa start tracking his former student Tomoe through Mongol archer positions and free some prisoners. It rewards the Charm of Efficiency and 2 Silk.
Dreams of Conquest
Dreams of Conquest is Ishikawa Tale 5 of 9 and the first of his Act 2 tales, starting at the Akashima Survivor Camp. You rescue a pinned-down Ishikawa and raid Tomoe's archer training camp for clues. It needs A New Horizon done and rewards the Charm of Precision and 2 Silk.
Laid to Rest
Laid to Rest is Ishikawa Tale 9 of 9, the Act 3 finale of his storyline. You team up with Tomoe and Ishikawa to stop a surprise Mongol attack on Umugi Cove. It rewards the Charm of Precision, the Midnight Shark half bow dye and the Shattered Mask of Tomoe.
The Ghost and the Demon Sensei
The Ghost and the Demon Sensei is Ishikawa Tale 6 of 9, an Act 2 side tale near Kawamata Village in Kushi. Tomoe has framed you and Ishikawa for killing travelers, and you hunt for her Kushi training camp. It rewards the Charm of Swift Return and 2 Silk.