"The Devil's In The Details" - How Mephisto Could Be Pulling The Strings In WandaVision
As previously mentioned Mephisto is better known for his relationship with Ghost Rider than any other character. He's the figure who damned Johnny Blaze's soul and forced the spirit Zarathos to do his bidding for an eternity, which led to Blaze bonding with Zarathos and becoming the Spirit of Vengeance.
However, to call Mephisto just a straight-up Ghost Rider villain would be to downplay his influence on the Marvel Universe at large. The ruler of Marvel's Netherworld has interfered with heroes and villains alike in the past, more recently making a nuisance of himself to Doctor Strange and Jane Foster's Valkyrie in her own comic.
But more important - and relevant - in 2021, is the demon's connection to Scarlet Witch. In the comics, Wanda uses Mephisto's soul to give birth to twins, who would later grow up to become the heroes Speed and Wiccan. However, Mephisto being Mephisto (and that's somewhere in between Futurama's Robot Devil and Tim Curry's Lord of Darkness from Legend, FYI), he reabsorbs his soul and by proxy kills Wanda's children.
It would take years for Wanda's children to return, reincarnated with different families, by which point Agatha Harkness - who many believe to be Agnes' alter-ego in WandaVision - cast a memory loss spell on Wanda to deal with the trauma.
That comics connection could prove vital to WandaVision's story, although it doesn't exactly clear up Mephisto's potential motivations. Agatha's own status as a mentor to Wanda also makes her role in WandaVision difficult to place. Is she creating the illusion to protect Wanda, similar to how she wiped away her knowledge of her children, or is she aiding Mephisto in a bid to control them?
The familiar chants of "for the children" in the second episode and Agnes reminding Wanda about school admissions definitely makes it seem as if she's manipulating her into creating the twins, although for what purpose remains unclear. Wanda still retains full control over the world she's created, but it's clear that there are other powerful forces at play
Either way, Wanda's role in the next Doctor Strange film is a big giveaway that this is only the beginning. Although Nightmare is heavily rumoured to be the villain of that picture - and SWORD's presence in WandaVision hints at an alien presence lurking in the wings - Mephisto should be considered a strong candidate for that film too.