The twin sons of Vision and the Scarlet Witch were born out of magic and the probability altering powers of Wanda. However, what really happened is Wanda's power latched hold of two shards of the demon Mephisto (he had recently been defeated and split into separate fractions by Franklin Richards). Agatha Harkness arrived after the twins began disappearing (they would vanish whenever Wanda wasn't thinking about them, i.e. during a desperate battle). Soon after Master Pandemonium kidnapped the twins and converted them back into their demonic nature for Mephisto. Though it appears that they never really had souls of their own, Kang (a time traveller) and Mantis have recently fought against the Avengers with two brothers in their twenties named Tobias and Malachi. They appear to be somehow connected to the Vision and Scarlet Witch, but whether they are William and Thomas remains to be seen.