At the beginning of the times, Aurelion Sol was born to give shape to the Universe. He traveled through the Universe, manipulating the energies of the cosmos to forge stars. At some point during his travels, he found a weird spot in the Universe. There, the energy was different. He didn’t care and forged a star using this energy.
Centuries later, he took a rest to see the work he had done. He discovered that around the stars he forged, world were created form stardust; in some of these worlds, there was even life. As he was approaching the weird spot he had found earlier, he saw that some of the worlds were shattered and, when he reached the spot, he discovered to his horror that the star he forged had become an immense monster: Bellomarus.
He fought this monster, but neither of them could defeat the other, so they both departed. For the next millennia, Aurelion Sol kept forging stars, while Bellomarus kept destroying the worlds that appeared around them. Every time their paths crossed, they battled.
At some point, Bellomarus discovered that Aurelion Sol disappeared. However, he didn’t feel victorious, he felt lonely. He tracked Aurelion Sol to Runeterra, where he had been fooled by the Targonians. He decided to help him recover his freedom, so they can fight each other again.