Crucible was another sibling of Princess Celestia. This older brother created the ground and molten core of Equestria as his sisters created the sky and the heavens. Being a giver of life, he brought to the Earth Ponies into being, imbuing them with great strength to compete with their cousins, the Pegasus and Unicorns.
After his work was complete, Crucible was concerned by Celestia's choice to teach and watch over the newly created lifeforms of Equestria. He believed as their parents before them, to allow only the ones worthy enough of ascension to earn their place amongst the Alicorns' ranks instead of being guided in, such a thing was thought to only breed weakness and lazy kin. One day, Crucible rose up in defiance of his sister after centuries of arguing and bickering, he grew impatient, enraged by Celestia's coddling of the ponies. He fought his much younger sister for control to restart Equestria as their parents saw fit in his eyes, unleashing a great Caldera Eruption, causing a great cataclysm. Though older and more experienced, his magic wasn't enough, his more powerful sibling defeated him with her own magic, casting down and banishing him to the very core of the planet after a long and drawn out battle.
It is said, the emergence of dragons and volcanoes are a sign of his still fuming rage and every time an eruption occurs, it is him awakening again to try and escape in vain, his magic and anger still very much intact however.