Wanda’s Encounter with Stark-tech Missile in "Previously On"
When recounting her and Pietro’s encounter with a Stark-tech missile in "Previously On," Wanda believes that the projectile malfunctioned. Agatha Harkness theorizes instead that Wanda used a "probability hex" on the bomb, allowing the twins to avoid the same fate as their parents.
Agatha’s Influence on Wanda
It’s possible that Agatha is using her powers to control Wanda’s powers, piggybacking off the Scarlet Witch’s immense magic to achieve her own ends. Agatha’s greatest desire is apparently to screw with Wanda, so the Darkhold may have given her instructions on how to blend her powers with Wanda’s to achieve her ends.
Conclusion of WandaVision
Literally and physically, Agatha Harkness is still alive at the end of the series. She fights it out with Wanda for most of the episode, trying to steal the Scarlet Witch’s powers for her own dark (albeit still somewhat nebulous) purposes.