Vecna was a Lich-god and one of the greatest evils ever seen on Oerth. Originally the wizardly ruler of a kingdom in the Flanaess, Vecna's rise to power was supposedly engineered by an entity known as the Serpent (which may have been Asmodeus, and is also said to have influenced much of his later activities). His terrible reign extended beyond his death, as he continued on as a lich.
Eventually, however, his most-trusted lieutenant, Kas the Destroyer, destroyed Vecna's physical form using an artifact, the Sword of Kas, which Vecna himself had created. Vecna left his infamous Eye and Hand of Vecna behind, as well as a host of lesser artifacts from his other body parts, all endowed with a portion of his might (and in the Eye and Hand's cases, a portion of his evil). However, Vecna was feared and venerated to such a degree that he ascended to demi-deityhood, and soon plotted against the other gods of Oerth to gain ultimate power over all the planet. As part of that plan, Vecna formalized his worship, touting himself as a god of secrets and knowledge who wished to free mortals from the ignorance imposed upon them by the other deities. His cultists made use of the monolithic complex known as Tovag Baragu to set up an interdimensional base.
Eventually, Vecna made his bid to take over Oerth by making himself the world's supreme being. He was only stopped by his rival Iuz and a band of adventurers. However, the resulting connection to the Prime Material Plane allowed the Dark Powers to entrap Vecna in Ravenloft. Once there, he was "granted" the domain of Cavitius, adjacent to Kas' domain of Tovag in the cluster domain known as the Burning Peaks. Vecna put on a show of frustration for the Dark Powers while he plotted his escape.
Eventually, he leaked information (via his cultists) to Iuz to trick him into travelling to Cavitius. Iuz was convinced he could ascend to full godhood by absorbing the power of Vecna; instead, Vecna absorbed his power. In the burst of energy that resulted, Vecna warped Ravenloft into a means of accessing the interplanar city of Sigil as a greater power. The godly presence in Sigil began to put stress on the structure of the multiverse, until a band of adventurers were able to destroy Vecna's body and cause his banishment from the City of Doors. Iuz separated from him as a result, but Vecna remained a lesser power of secrets, and continues to gain worshippers on Oerth.
The name "Vecna" is an anagram of "Vance," as in Jack Vance, a fantasy author whose works influenced the design of Dungeons and Dragons.