After you find the treasure in Misko's Cave of Chests, you find a message in a bottle with hints to a Fierce Deity treasure.

Quest Location

Complete Misko's Cave of Chests and examine the bottle to start this quest.

Beneath the Bedchamber of Akkala's Red-crowned Citadel

Go to the top of the Akkala Citadel Ruins, west of Tarrey Town. Explore the ruins until you find an enclosed room that seems to have no entrance. In one corner of the building, there is a small opening that you can crouch to go into. In the room, you can fall through the hole to enter the Akkala Citadel Ruins Summit Cave. Open the chest there to get the Fierce Deity Armor.

In the Skull's Left Eye

Go to the tall spire in one of the eyes of Skull Lake, which is northeast of Death Mountain. There is a hole in the top of the spire that you can fall into. There is a piece of rubble from a sky island at 3220, 3626, 0218 (northwest of Skull Lake) that you can stand on and use Recall on, then when it stops going up, you can glide over the opening in the spire and go down from there. When you fall into the hole in the top of the Skull Lake Spire, you will reach Skull Lake Cave. Inside the cave, you will be attacked by Stalkoblins, Stalmoblins, and a Stalfos. On top of one of the platforms in this area, you find a chest. Open it to get the Fierce Deity Mask.

In an Old Stump in Hyrule Field

Southwest of Lookout Landing, you can find Mount Daphnes. North of Mount Daphnes, you will find the Ancient Tree Stump in a lake. Fall into the tree stump and you will be in Ancient Tree Stump Cave. At the bottom, look for a tunnel in the north wall with vines covering it. Cut through the vines, and cut through the next set of vines, and open the chest there to get the Fierce Deity Boots.

Return to Misko's Cave of Chests

Go to the quest destination on the map, north of Kisinona Shrine and Foothill Stable. Equip all three pieces of the Fierce Deity armor set, then enter Cephla Lake Cave, to the area where you found the bottle. A wall will slide open, revealing a chest. Open it to get the Fierce Deity Sword.