Heiress to the house of Gremory; Switch Princess; Buxom President of the Occult Research Club. These were the words to describe Rias Gremory, but she would come to another name, "Devil Planet."
It happened without no knowing what was gonna happen before she became a Living Planet, except for Rias who saw it in a dream.
It all started when through Issei's death that Rias was revived. Still a Buxom Devil with now similar bust, waist, and hips to Akeno's, but without a husband to wed. She slept in her lingerie experiencing the same dream that Issei would give her a second chance to ensure the victory over the monster who destroyed the Earth with a promise that she will be grant with something to honor him. That pained Rias for so long that she would stab herself with a knife to be with him.
Rias undressed to her lingerie, ready for that dream again, went to sleep. However, Rias had a different dream. This time she saw a beautiful goddess. Remembering her repeated dream, she knew that she would have to honor Issei in some way, but what?
Rias: Uh, hello.
?: Hello, Rias. I am here to grant you something to honor your dead boyfriend through his final wish for you.
Rias: Issei had a final wish for me? What is it?
?: You will grow so much the Earth will be in between your butt cheeks like a planetary canyon.
Rias: Planetary Canyon? Wait. Is Issei's wish to make me into...?
?: Yes, a living planet.
Rias: When you wake up you will start to grow in your lingerie until your the Earth in behind the middle of your butt.
When she woke up, she remembered the dream she had and started to grow with her lingerie growing with her. She continued to grow and grow, even after she grew out of her house. She still kept growing. Suddenly, Rias had an idea.
Rias: I think I'll walk while I grow.
And so she walked. With each step she took, she grew bigger and bigger, shaking civilization, forests, savannas, jungles, she walked around the world. Then when she got too big, she jumped off, not wanting her new inhabitants to be crushed with the Earth and went in front of it. She grew and grew until the Earth was in the middle of her butt.
Rias: Ara ara. I'm a living planet! *gasp* And everyone's moving to every part of my body! The Americas are moving to my boobs for skin and milk! The Africans are moving to my belly button for clothing! And the Europeans and Asians are moving to my butt for every single touch of my planetary canyon! Ara ara! I'm a living planet! Maybe I'll take this opportunity to unify the universe. A planetary quest for a planetary goddess. And I have my inhabitants come along for the ride.
So the now planetary Rias set out to her destiny as a planetary goddess. A universal unification.