The second deity to learn that the true power of the universe lay within the Astral sea, Vecna gathered all the knowledge and power that he could, forcing Ioun into a race to control the primary source of all magic. It was not Ioun’s choice, but had she not competed, Vecna could have become the dominant power in the cosmos. It was in this conflict that the Shadowfel and Feywild were created, each being used as a repository for power siphoned out of the Astral sea. Vecna and Ioun remain locked in an eternal struggle of wills, and in this balance magic is accessible to both mortals and the other gods.
God of Necromancy and Secrets
Holy City: The Scarlet Abattoir
Legend says that before Time Immemorial Vecna was a human wizard, but he grew so powerful from stealing the souls of his adversaries that eventually he ascended to godhood. Vecna represents everything the Raven Queen hates, an unnatural continuation of the dead. That is probably why she pulled out his eye in the War of Entities.
Vecna is always up to something, and so are his clerics. Beware a hostile priest of Vecna, but beware a helpful priest of Vecna even more.