Promise to keep it safe

So I have seen a few theories going around but I'm in my re-read of OS and wanted to make a post specifically about the "IT" Xaden refers to in his letter at the end of OS.

In CH 6, Xaden and Violet are talking alone in the gym, he says to her, “The only solution I can think of is you. You’ll be the first to sense when I lose the rest of what makes me…me.” they go back and forth for a bit and then he says, “Swear you’ll sound the alarm if I go too far, that you’ll keep it safe, even if it’s from me.”

She promises him that she'll keep "it" safe. This is before she knows he got his title back and a seat in the Senarium, so he can't be asking her to keep his title safe. Xaden and others refer to Tyrrender as she/her, and again, it's before she learns he got his title back, so he doesn't mean Tyrrendor either.

So what is the "IT" she promises to keep safe. What "it" is now Violet's responsibility at the end?

One person in a comment on another post said Xaden might mean his soul and probably made a horcrux or something, which I love the idea of as a Harry Potter fan.

I'm thinking maybe some type of rune? Maybe her wedding ring is a rune & Riorson family secret that she has to keep safe?

What are yall thinking?