Athear Austrat (Celestial Flight)
A former assassin racked with guilt over those he's killed, turned to holy priest of Garl Glittergold.
The whisper Gnome known as laughing Shadow because more and more attuned to the shadows as he grew in power. In the final conflict with Vecna, no one really knows if he made it out alive or not.
Revealed hundreds of years later to have become the new Vecna. Helps the party figure out what is going on, guides and protects the party.