Who is Vecna 001?

The final scene of Volume 1 shows a shot of Vecna, and a close up of the 001 tattoo on his arm, confirming that Henry Creel, a.k.a. Number One, became Vecna. 07-Jul-2022

https://www.marieclaire.com/culture/tv-shows/vecna-stranger-things-explained/#:~:text=The%20final%20scene%20of%20Volume,a.k.a.%20Number%20One%2C%20became%20Vecna.