At the end of iron island there is a dark door(you have to have beaten the elite 4)
Registeels' cave is on Route 120. Once you get near the lady who gives out berries, go up the stairs, through the long grass, and up another set of stairs. You should now be at Registeels' tomb.
you go to aegis cave in registeels room and you go to kick butt.
either left of lilycove, sandstorm bit or water near dewford
Ruby and sapphire:with new time hope and love aim to the sky in middle, which means use fly in the middle of the cave. Emerald:those who inherit our will, shine in the middle, which means use flash in the middle of the cave.
in the sky
Go into the middle of the room and use FLY (like if you were going to fly to a different town) That should open up the braille door to the north, and you can fight Registeel!
south of fortree west of lilycove
Go inside Registeels tomb(after going to sealed chamber.) Read the wall. It says "To those who inherit (something), shine in the middle. Translation: Use flash in the middle of the cave.
You need to first do the underwater morose code thing in the currents west of pacidflog town. You need a wailord and relicanth to do it though. You also need dig. After you do that cave, the 3 entrances to the 3 regis should be open.
Go to the middle of the room and use flash
use flash in the middle of the tomb
Get nat dex then you must get regi rock regi ice and regi steel for it to appear