Quest to make it stop raining on one of the islands
Orobashi was a serpent god that was slain by Raiden around the archon war. Orobashi was then buried and sealed on Yashiori island. Recently, the Sangonomiya rebels destroyed the wards sealing Orobashi. This caused the Tatarigami to spread across the island, making it rain and thunder in perpetuity.
When the traveler meets Kaji, he asks if they can help repair the wards on the islands to combat the Tatarigami. The sacred stone mounds and thunderbearer mirrors can be used to destroy the barriers created by the Tatarigami.
At the end of the questline, you get the Kitain spear crafting diagram as a reward.
After completing the Inazuma archon quest, head behind the house at the South of Ritou island. There is a hatchet. Interact with it by pressing 'View'. This will transport you to a cove by a beach. Pick up three items, then head by the beach to wait for Harrison to arrive. Harrison gets ambushed by electro slimes. Defeat them and complete the quest.
Gendou sells fortune slips at the Narukami shrine. Interact with her the first time and draw a fortune slip. Interacting with a tattered note by a Fatui camp around Araumi starts this hidden quest. Over the course of 4 days, visit Gendou and draw fortune slips in the middle of a hazy night. Then go to specific locations around Inazuma to defeat Fatui.
The Fatui have sent a spy Lyudochka to act as an imposter for Gendou. Momoyo is a ninja. We gain access to the secret dungeon hideout at the base of Mt. Yougou. The traveler applies chemicals on Lyudochka so that they can follow her elemental trace to her secret meeting with the Fatui.
The Fatui plan to sow discord between Watatsumi and the Shogunate. But the traveler infiltrates their ranks and foils their plan. Momoyo gets captured by the Fatui and starts wearing an eyepatch after some toxic chemicals get into her eye.
We enter a hut around Araumi, beat up the Fatui Pavel and Ilyin, and rescue Momoyo.