The secrets in the Pokémon Sword and Shield’s Crown Tundra region comes with several mysteries for you to uncover. They take a bit of thinking to figure out, but if you’re able to make it to the end of each of the tiny adventures, you can receive a legendary Pokémon for your trouble. There are several Regi Pokémon with temples all over the region, and the one to catch Registeel is right outside of Freezington.
The entrance to Registeel’s temple is to the south of Freezington, in the Giant’s Bed region. It’s on the west side of the area. It’s not too difficult to find when you fly into town and ride down the hill. When you arrive at it, there will be a Watt Trader there you can fly to, serving as an ideal checkpoint for you to mark where you need to go.
When you approach Registeel’s entrance, you won’t be able to open it. The entrance requires for you to, “Let ring the piercing note that will wake the giant of steel.” The other Regi Pokémon entrance has similar requirements, but this one is a bit vaguer. The hint is referring to a feature of Pokémon Sword and Shield where you can whistle. To whistle, merely click the left joystick button standing outside of the entrance. You want to face the entrance when you do it. After you successfully whistle, the doors to the temple open for you.
Inside, there will be various dots on the floor, representing Registeel. You now need to walk across all of the dots to light them up, and the presence of the air will change. Approach the statue towards the back of the temple, interact with it, and you can battle Registeel to capture it.