This is one of the harder shrine quests in the game, with multiple steps and zero hand holding – so read on for our guide.
You can find Kass in the Pagos Woods, east of Hylia Lake. If you've already found the Pumaag Nitae shrine, fast travel there and head east.
The song he sings you contains this riddle: "Where the forest dragon splays its jaws, a shrine sleeps with noble cause."
He also mentions that the forest in the riddle could be the Damel Forest to the north. Sure enough, if you open your map and look next to the Damel Forest, you can see this river, which looks a bit like something opening its jaws ... though perhaps not a dragon.
You'll find an enemy camp there, with plenty of Lizalfos and a black Moblin to take care of. Head inside, grabbing the 300 rupees from the chest on the way in, and find a Spring of Courage which tells you: "Offer Farosh's scale received from the golden spirit to the Spring of Courage."
Where to find Farosh's scale
Head west to Lake Hylia and wait for the night. Farosh emerges from the lake at 1:30am and heads toward the south bridge tower, so climb up there before it starts raining.
Equip your bow and land an arrow on Farosh's body to knock off a scale. It'll likely land in the water, but don't worry – it glows enough that it's easy to spot.
Grab the scale, head back to the Spring of Courage and drop it in the water to open the door, revealing the Shae Katha Shrine.
Like many of the other secret shrines, this is a blessing – so just grab the Thunderspear from the chest and you're all done.
Looking for the other secret shrines in Breath of the Wild? We've got you covered.
