Pokémon Ruby and Sapphire/Cave of Origin

In Sootopolis City the weather has gone awry. If you are playing Ruby, is causing strong sunshine, or, in Sapphire,  is causing heavy rain. You must stop this.

Go to the northwest of Sootopolis City. Steven will meet you and introduce you to the Gym Leader Wallace. on seeing the Orb that you're holding (Red Orb in Ruby and blue Orb in Sapphire), Wallace takes you to the Cave of Origin.

The Cave
It is dark inside the cave so bring a Pokémon that knows Flash to light it up. The cave is shaking due to the roars of the legendary beast.