Why did Will Byers suddenly have the ability of Vecna? No one realized that Vecna was playing game?

Why did Will Byers suddenly have the ability of Vecna? No one realized that Vecna was playing a huge game of chess! #strangerthings #willbyers#usa #fyp #foryou#strangerthings5#hbo #didyouknow #strangerthings4 #strangerthingsfanart