When the fallen angel Lucemon was defeated by the Legendary Warriors ages ago, rulership of the Digital World fell to the three Celestial Angels. These powerful beings guided the Digital World wisely for millenia (Digital World time), and called upon the powers of the Royal Knights when needed.
Seraphimon was the leader of the Celestials, and was called the Lawgiver and the Celestial of Hope. He guarded the Spirits of Fire and Light.
Ophanimon was called the Celestial of Light, and dedicated to peace and love. She guarded the Spirits of Ice, Wind, and Thunder.
Cherubimon was called the Celestial of Wisdom, and was the lore keeper. He guarded the Spirits of Darkness, Metal, Wood, Earth, and Water.
But eventually, lingering resentment poisoned their once close bond. Cherubimon started to feel slighted, as the only Beast Digimon of the Celestials, and by what might be construed as a romantic jealousy of Seraphimon. He was easy prey, then, for the dark power of Lucemon, as the ancient horror reawakened.
Cherubimon, darkened by Lucemon's power, attacked Seraphimon by surprise and sealed him away. Ophanimon bartered for his life and in the process became Cherubimon's prisoner. But she had a plan--a plan that involved reaching toward Earth, and the Spirits she and Seraphimon guarded...
In the process of the events that unfolded, all three Celestials were destroyed and reincarnated. It is unknown if Patamon, Salamon, and Lopmon will one day reassume their duties.