Spoilers for Return to the Tomb of Horrors, Vecna Lives! Vecna Reborn! and Die, Vecna, Die!
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Vecna was already a demigod due to his worshippers waaay back in Vecna Lives! He hatched a plan to become the only Greater God on Greyhawk, through the powers of the Tovag Baragu stone circle. Thanks to timely intervention by Iuz, he was banished (along with Iuz).
He next shows up in Ravenloft leading armies eternally against Kas in his own little Prisoner domain, attached to Kas's little prisoner domain. Vecna hatches another plan (sucessfully!) to escape his domain. To my knowledge, he is the ONLY dark power to ever successfully escape his domain.
He THEN comes up with a plan to enter Sigil (Planescape setting). He kicks the Lady of Pain's butt (sacrilege to many gamers, I know), and (I'm a little fuzzy on this part) successfully uses the power gained to re-enter Greyhawk, only as a Greater God. This is the position he takes in the Pantheon as of the 3E PHB.
Vecna as written makes the ambitions of Elminster and Larloch (or Forgotten Realms) look petty. The only former mortal who rivals Vecna's level of ambition is Acererak, from Return to the Tomb of Horrors, who strove to BECOME the Negative Material Plane.
Darklone, which Ravenloft book are you referring to? The 2E or 3E version? If they still have him in the 3E book, then someone at Sword and Sorcery may be ret-conning the "Vecna!" series. I tend to believe the PHB, myself.
