With a ton of power behind it, TM23 Steel Tail in Pokémon Shining Diamond and Shining Pearl is a sought-after attack for those who want to strike fear into ground-type users. Can be found in an optional end-of-game area.
TM23 Steel Tail is located on Iron Island, which can be accessed by ship from Canalave City. Go south of the gym in that area, pass two houses and you will see a sailor ready to take you to Iron Island. Set sail and you will soon be at your destination.
After climbing a few stairs when you arrive, enter the cave. When you enter, take the ladder on the right down into the depths. Run the path up, go down the steps and keep following the path until you see a trainer in a green hood. Pass it and turn right. Now, take the brown elevator down. Go right again.
Then, take the downhill path and go up the narrow steps. Follow the path and then to the right of the patrol worker you will see the MT23 Iron Tail waiting to be collected.
You'll be thankful you grabbed Iron Tail from Iron Island. This steel move features a strong power stat of 100 but lacks accuracy slightly at a speed of 75. With this little caveat in mind, it also has the ability to lower an opponent's defense when used. You may want to use this attack on a formidable Pokémon of steel like Steelix.