Use the following steps to capture the three mysterious Pokemon called the Regis. Go to the southeast part of Route 134 and dive into the water. It is difficult to get to due to the currents, and it is the only deep part of that route. You will find a cave. At a part of the cave, there are some Brail writing on the wall. At that point, float to the top of the water, where you will end up in a cavern. Go into the last room by going forward, use Dig, and in the next room you must do something strange. Put a Relicanth in the front of your party, and a Wailord at the last place. Walk to the top of the room and read the Brail . You will trigger an earthquake, which will unlock the Regis. You can fish up a Wailmer to evolve at level 40 at Route 122, and find a Relicanth outside Sootopolis in the underwater grass (rarely, but it is there).

To find Registeel, you can access a volcano in the southwest region of Route 120. Enter it, then move towards the center of the room. Use Fly to find Registeel.

To find Regirock, fly to lavaridge, bop down the mountain, and enter the desert. In the southern part of the desert, you will find a cavern. Enter it, then walk up to the center tile at the top of the room. Go three steps down and two steps right. Use strength to open the passage to Regirock.

To find Regice, in Route 105, there is an island on the west side that has a mountain on it. Go there, enter, and stand in front of the tile. Press A, read it, press A again, and do not do anything else. The door will open after twenty seconds, and the last Regi is yours.

You never have to use the Master Ball to catch all the legendary Pokemon. You can catch them all with about 60 Great Balls. However, you might need to use a Master Ball against the last one in the tower.