Charlotte had a rough idea of what the "key" Guardian's Staff was referring to.
As the source of Harald's Bloodline powers and an existence with a higher status than the Divine Artifact of the Holy Royal Court, there was only one such item in Charlotte's possession, deeply integrated with her—the Blood Holy Scripture.
As for why Harald knew about the Blood Holy Scripture, considering his relationship with Lilith, and since the Blood Holy Scripture was crafted by Lilith, it was no secret to Harald.
Of course, following the current course of history, when Charlotte and Lilith's positions are fully integrated, Harald would probably view the Blood Holy Scripture as something crafted by Charlotte herself.
Regardless, leaving this as a contingency plan was indeed clever.
Exactly, a contingency.
Charlotte was confident that the so-called "key" was definitely Harald's contingency plan.
