i'm waiting for this chapter: seb has to find a way to defeat ut, so he (or maybe ociel?) pretends to destroy the lockets (his weak spot, as far as they know), ut loses his shit and gets finally defeated by seb. you think it's plausible?
Kind of fits in with part of Mother3 theory, since I said the lockets are a parallel to the Courage Badge, which turns out to be the Franklin Badge. In Mother3, wearing it saves Lucas, since certain attacks bounce right off of it, like… I think it’s called PK Flash or something? I haven’t thought about the attacks in quite a while. Except that Lucas usually uses PK Love. (There are also huge parallels to Harry Potter, but that’s a series of posts I’ve yet to properly write….)
Anyway, I theorize that they don’t think about the lockets too much and don’t know their true worth… to Undertaker or to themselves, but when the twins have their final confrontation, those lockets become important again. I don’t know exactly how they come back into play. There are some options; here are three:
Undertaker has such an emotional bond to the lockets that breaking them “breaks” him. That’s pretty much what you suggest in your ask. (It’s a bit like destroying the horcruxes in HP, even though Undertaker is not a direct parallel to Voldemort.)
The lockets have an unrevealed use, like containing the souls of the people they represent. See, in Mother3, the ghost of their dead mother, Hinawa, is what finally gets Claus to stop fighting his younger mirror twin, Lucas. Then Claus aims a final attack at the Franklin Badge — PK Flash, I think — and that is what finally destroys him… who had been turned into a Fascinating Chimera after he had essentially died. (Items containing souls/soul shards and releasing them to finally destroy a “twin” brought back to life goes back to HP, too. 👀 😏 Recall that Harry and Voldemort have “twin wands”, and ghosts from Harry’s past show up to give him pep talks, and…. Yeah.) Now, Hinawa isn’t in the Franklin Badge, but the combination of her ghost showing up and that badge being there are the key to Lucas winning the battle, which ends up being mostly psychological.
The lockets save our earl in a more physical way; just like the Franklin Badge reflecting a PK Flash and saving Lucas just to destroy Claus… perhaps a locket blocks a bullet. Maybe the bullet even ricochets?
I expect our earl to not really want to fight this bizarre doll version of Ciel, not in person, because even though real Ciel is now hardly the brother our earl once knew (soul is gone), he still looks the part, as he has the body and the memories. Like in Mother3, I expect real Ciel to be more willing to physically fight our earl than our earl is willing to physically fight real Ciel. Because Fascinating Chimera Claus is “heartless”, just like Bizarre Doll real Ciel is soulless.
I expect Sebastian to be too busy dealing with Undertaker, as well as possibly Polaris and the other lords of the stars, to be able to help break up any fight directly between the twins. A bit like how Grelle was keeping Sebastian busy while Madam Red and our earl faced off. Madam Red went in for the attack but finally hesitated at the last moment. Meanwhile our earl completely hesitated to shoot his aunt; he didn’t even grab his hidden gun. But this time, real Ciel might not hesitate at all, and idk if our earl has the guts to shoot back. 🧐
And that’s kind of where the lockets might come into play. It could be as simple as a locket physically stopping a bullet from killing our earl. Or it could be turned into something way more complicated.
Imagine this crazy scenario:
Sebastian is yelling for his young master to fight back against real Ciel, but our earl is hesitant. Real Ciel aims a pistol at our earl and fires. The bullet hits the locket of their grandmother. Her soul is released, and it somehow manifests itself in a way they can perceive — maybe because they are both part reaper? — and she tells them they shouldn’t be fighting each other. She might also give Undertaker a stern talking to for putting his children and grandchildren through so much pain and emotional trauma. Perhaps she disapproves of all the experimentation he’s done? You get the idea. Then the twins stop fighting, and Undertaker finally stops, too. Granny Phantomhive convinces real Ciel to join her (just like Hinawa’s ghost said Claus must be tired and should join her). Perhaps real Ciel even decides to bring his own existence to a final end, just like Claus does.
And Undertaker parallels a few characters in Mother3 — two who live and one who dies — and Snape (who dies) in HP, so it’s possible he either gets destroyed… or gets captured and returned to the reaper organization.