You enter the Oman Au Shrine, where you gain a new ability for your Sheikah Slate.
How to Get the Magnesis Rune
Check on the pedestal in the first room of the shrine. The Magnesis Rune will be added to your Sheikah Slate.
How to Use the Magnesis Rune
Press L to activate the rune. Anything glowing bright pink can be picked up and moved around. Aim the reticle at a pink item to make it glow yellow. Press A to pick it up. (If it doesn't glow yellow, it can't be picked up.) You can't pick up items that you are standing on. You can use motion controls to move the item, or you can use the left control stick to move the item up or down and to the sides. Use the directional pad to move the item away from you or toward you.
Reach Oman Au
Now pick up and move the metal plate closest to the pedestal where you got the rune. Go down the ladder in the hole that is revealed. At the end of the path, go up the stairs. You can use Magnesis on the metal block in the wall to push or pull it. Use the metal block to push the other blocks out of the way. There is a second metal block in this room if you need it.
After going past where the wall of blocks was, a Guardian will attack. Hide behind a wall if you can. Drop a metallic object on the Guardian to damage it a lot. Or just go up to it and attack with a weapon. But stay close to walls so you can hide behind them if the Guardian is about to blast you with a laser.
Then walk across the flat metal slab, then use Magnesis to move it and create a bridge to the next platform. Then use Magnesis on the treasure chest on the ledge to the left. Pull it toward you and open it to get a Traveler's Bow and five arrows.
After that, pull on the metal doors to swing them open. Then go up the stairs and check on the glowing blue box. You will get a Spirit Orb. It will be useful later.
After this, you will automatically go back outside, where the old man will talk to you.