In The Legend of Zelda: Tears of the Kingdom, the Simosiwak Shrine can be found on Bravery Island in the North Hyrule Sky Archipelago above the Great Hyrule Forest.
For this shrine, you’ll need to complete the Proving Grounds: Lights Out challenge where you’ll be unable to use any equipment from outside the shrine and the whole challenge will be done in the dark. When you start, head down the stairs and look to the left to find some weapons. You’ll then need to head into the darkness and go left to find a lone Construct to fight. Use some of the materials it drops to fuse with your weapons to make them stronger.
Next, head to the other side of the room to find an enemy wielding a Ruby Broadsword which shoots fireballs. After you take this Construct out there will only be one left. This one also has a weapon that emits flames. After you defeat this one, the challenge will complete. Head to the altar to claim the Light of Blessing. Just before the altar, open the chest containing a Bright Elixir!