The Blood Sutra is Norio Tale 5 of 9, an Act 2 side tale where you search for Kushi Temple's missing abbot. The trail runs from Koshimizu Farmstead to a waterfall. It rewards the Charm of Resistance II and 2 Silk.
Part 5 of 9 in Norio's Tales
Part 19 of 66 in Tales of Tsushima
After the attack on Kushi Temple, the abbot is missing. Norio thinks he may have taken shelter at Koshimizu Farmstead, so you ride there together. Check the fire, then search the house. He is not there, but you find tracks. Follow a trail of blood to a group of Mongols, fight them, then keep searching the area until it leads down to a waterfall.
Talk to Norio and ride with him to Koshimizu. Check the fire and search the house. Find and follow the tracks. Beat the Mongols at the end of the trail. Search the area, then the next search zone. Check the waterfall below and the body there.
The search ends at a monk's body below the waterfall. Later dialogue confirms the Mongols killed Abbot Komei, leaving Kushi Temple without its leader just as the monks are losing heart.
The Last Warrior Monk
The Last Warrior Monk is Norio Tale 1 of 9, the Act 2 side tale that opens his storyline at Akashima Village. You help Norio track down monks the Mongols abducted from the village. It needs A New Horizon done and rewards the Charm of Resistance II and 2 Silk.
The Birth of Suffering
The Birth of Suffering is Norio Tale 4 of 9, a short Act 2 side tale where you arrive at Kushi Temple to find it under Mongol attack. It is one straight fight alongside Norio and the temple monks, and it only rewards a minor Legend increase.
The Cause of Suffering
The Cause of Suffering is Norio Tale 6 of 9, an Act 2 side tale where you recover Kushi Temple's stolen Buddha statue from a Mongol camp. It starts at Kushi Temple and rewards the Charm of Well Being II and 2 Silk.
The End of Suffering
The End of Suffering is Norio Tale 7 of 9 and the last of his Act 2 tales, where you protect Kushi Temple's monks from another Mongol attack. It rewards the Charm of Fortitude and the Clear Summer Headband.