Okay so this guy won the poll. In my version of the story, the pale king had a royal scientist of sorts named Troia. This guy is a proper heartless sadist whose job it was to see if you could *make* something hollow. Usually with that sort of thing, you either are or aren’t. But with the hollow knight/pure vessel, its feelings and desire to save hallownest was what led it to not being hollow, so it must in some part be cognitive. You might see where this is going. He wanted to see if mentally fucking up a non-hollow vessel enough would make it hollow through that, make it give up on feelings and desires, make it empty. Even after the pale kings death, he continued the experiments with the few remaining vessels.
One such as Acirum.
@palisaded