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.
Part 6 of 9 in Norio's Tales
Part 21 of 66 in Tales of Tsushima
The Mongols have carried off the temple's Buddha statue, and the monks have lost hope. You and Norio promise to bring it back. Talk to a monk at the temple, then travel with Norio. On the road you find a dying monk who points you east to the Mongol camp. Find the thieves, clear the camp and search it for the statue.
Talk to a temple monk. Ride with Norio. Talk to the dying monk, then Norio. Track down the thieves. Clear their camp. Find the Buddha statue.
You do get the statue back, but the Mongols attack Kushi Temple again in the following tale and the statue is destroyed in that fight.
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 Blood Sutra
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.
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.
Ghosts in the Fog
Ghosts in the Fog is Norio Tale 3 of 9, an Act 2 side tale where you defend Akashima Village from a Mongol counterattack using incense burners as traps. It rewards the Charm of Unyielding II, the Noble Fighter's headband and 2 Silk.