I had seen Agatha Harkness thrown around as a possibility, but I have no knowledge of the character, so just "Oh it's Agatha Harkness!" had no impact on me. Like oh okay. But luckily that's what the theme song is for, and I look forward to seeing what she's gonna do next week. Also yay Darkhold from Agents of SHIELD!
I had also seen people speculate about Mephisto or some House of M comic storyline as possible explanations too. I'm not familiar with those either, but who knows, maybe it's not too late for all of the above?