Silas. He was a witch and the first immortal being. He came up with spell with Qetsiyah. But he was cheating on her, so she sealed him and the cure. The only way to get to the cure is by waking him up.

But if he woke up, he would bring back every single supernatural being that ever died.

[Including a lot of very angry and pissed off vampires, werewolves, and hybrids. Sure, it would bring back a few people she wouldn't mind seeing again like Bonnie's Grams or Alaric or even Jeremy, but that little good does not outweigh putting the whole world at risk.]

We didn't think it was real. We thought it was just a myth that got spread around like the Curse of the Sun and the Moon. Not even most of the Originals believed it was true.