Time for a history lesson.
In 1985, writer Steve Englehart–known for weirdness–and artist Richard Howell teamed up on what Marvel then called a “Maxi-Series.” Twelve issues of the duo, off on their own. In issue #3, knowing that the synthezoid Vision was unable to create sperm, Wanda used magic, with the help of good old Agatha Harkness.
(Remember, Agatha taught Wanda how to use her magic powers way back in Avengers #128 (also written by Englehart).
Of course, magic babies never end well.
But before disaster struck, Vision was the proud daddy of not one but two hex-children….Delivered by Doctor Strange.
It ended tragically, of course. In the pages of West Coast Avengers, Agatha told Wanda that the children–Tommy and Billy–were unnatural children of magic and didn’t exist when Wanda wasn’t around. An extremely cool symbolic parent-role.
Anyway, Wanda went to join her teammates in a battle against Master Pandemonium, and in her absence MP killed them and absorbed all of Wanda’s memories about the children.
Now, you know what you need to know to read Avengers #500.