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.
Part 4 of 9 in Norio's Tales
Part 15 of 66 in Tales of Tsushima
Norio asked you to meet him at Kushi Temple. When you get there the Mongols are already attacking it, in revenge for freeing Hochi. Jump in and fight alongside Norio and the monks. Some monks die before you arrive, but the two of you push the Mongols back before the temple is lost.
Go to Kushi Temple to meet Norio. Beat the attacking Mongols.
This opens the Kushi Temple chapter of Norio's story, which runs through the next three tales. The temple's monks are not fighters, and over these parts you see whether they will stand up to the Mongols.
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 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.
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.