Vecna is a NE lich lesser god of secrets. He goes by several nicknames, The Maimed King, The Whispered One, etc. Legend tells that he can hear whenever his name his mentioned, and he knows any secrets that occur on Greyhawk. He secretly wishes to convert all of th multiverse to his worship and supplant all the other gods. Vecna is (or was once) opposed by Kas, formerly one of Vecna's most trusted henchmen who ended up betraying the Lord of Secrets.
You can find more info on him in Deities and Demigods. If you're interested and your DM lets you, you look at the 2E products Vecna Lives, Vecna Reborn, and Die Vecna Die!, which detail Vecna's return, imprisonment in Ravenloft, and his escape. I'm not familiar with the 1E products that detail him. There was a also a comic book Hand of the Revenant) that explored some of his background on ancient Oerth.
Edit: Some people disregard those 2E products I mentioned as junk because of some of the rules that they break.
Also, any info that you could find on his artifacts, the Hand and Eye Vecna, might be useful. These were originally parts of his body but he lost them as a mortal. Many would-be users of these artifacts have turned into madmen deluded into thinking they were Vecna himself.
The Return of the Head of Vecna! 
