Finding the King Dorephan Location is part of Main Quest: Clues to the Sky in The Legend of Zelda: Tears of the Kingdom. This guide shows where to find King Dorephan.
Step 1: Get Clue from Kids in Throne Room
The game will put a quest marker at the cave in Toto Lake, but this is misleading as King Dorephan isn’t found there. We must find him ourselves, the game doesn’t mark his location.
First, head back to Zora’s Domain. Go upstairs into the throne room. There will be some kids playing in front of the throne. They will send you away when you approach. You must climb the side of the throne to get behind them.
Then approach the kids from behind and the 
Listen prompt will come up.
Listen to them to get clues to King Dorephan’s location. They reveal that he’s in a cave behind a waterfall between Ploymus Mountain and Zora’s domain.
Step 2: Drop down Lulu Lake Waterfall
King Dorephan’s location is at Lulu Lake. Drop down into the cave next to the waterfall. You can glide to Lulu Lake from Ihen-a Shrine.
Step 3: Talk to King Dorephan
After dropping down the cave you will reach Pristine Sanctum. Talk to King Dorephan on his throne here. He will give you 5x King’s Scale, which you need for the last step of this quest.
That’s all there is to finding King Dorephan in Zelda TotK.
For everything else needed to obtain 100% Completion refer to Zelda Tears of the Kingdom Wiki & Strategy Guide.
Okay so I found the cave only problem is I fell to the bottom and now I can’t get to the strong because the walls are too slippery to climb so what do I do?
Use fast travel to a nearby shrine/skyview tower.