A/N: this is just a theory, please let me know if you think I forgot something or if there is a point that’s not clear. Very long post, heavy with spoilers.

First of all, I think we can all agree that, whoever the mastermind is, there isn’t any doubt that they are a supernatural character…

…And narratively speaking, after the Campania and Weston arcs, it’s indeed logical for both the readers and the characters to initially assume that the most likely suspect when the dead is brought back to life

(here: real!Ciel aka Lord Sirius, as well as maybe the other Lords)

is UT himself.

However, are we really sure at this point that Yana will go for the same ploy for the third time? Well, obviously this theory is here to propose that Yana won’t play the predictable card after such a complex and long arc. 🙂 Onto the hints:

1. A different motus operandi…
   
2. Different “bizarre dolls”…
   
3. …Thus a different mastermind?
   
4. real!Ciel vs our!Ciel
   
5. UT in ch108 & ch120
6. Additional links

1) A different motus operandi…

…Which for now creates a plot hole if UT is the mastermind, because what we were shown of the blue sect’s method (= blood transfusions + ???) doesn’t fit with UT’s previous technique to bring back the dead

during the Campania (= editing cinematic records with fake memories) and Weston (= editing CRs with “episodes” aka yearning for the future) arcs.

One argument would be to say that he changed his method of editing cinematic records to base his project on blood transfusions (+ something else) instead after Weston, but…

…He seemed to find Ryan’s attempt of using science to bring back the dead laughable during the Campania arc, so for now it’s hard to think he would have ended up using science as well. 

Another argument I’ve seen would be to say that the BDs we saw during the Campania/Weston arcs were from old experiments and that he actually changed his method a while back, but as brilliantly put by @thedarkestcrow [x]:

If he had known all along how to make a perfect Bizarre Doll, why did he wait so long? And why would he risk being found out and risk that the twin (if he really hid him) would be found by creating the failed Bizarre Dolls? No one suspected Undertaker of anything until the Campania arc so even if he did indeed hide the twin it may have been much easier if he had hidden his true identity and his experiments, too.

Besides, as pointed out by @hitsugikuro​, assuming that Othello and UT do know each other (”50 years ago”)…

…it’s possible that Othello was speaking of UT in ch119 when he mentioned someone (a deserting Shinigami?) who wasn’t “very knowledgeable in this [science] field”, thus reminding us possibly once more of UT’s opinion about bringing back the dead through science and crossing him out of the suspect list.

Secondly, there is also the fact that the blue sect has been existing for at least a year and a half, because we know that Bravat was regularly transfusing old Lords from the Parliament until recently…

…which indicates a switch in the sect’s purpose

(researching about blood transfusion and helping the old Lords => bringing back the “star Lords”)

from the moment Bravat opened the Sphere Music Hall and used the ex-prefects to attract the masses & start massive blood collects during Summer 1889.

Of course, UT could have been a part of them but, again, he doesn’t seem to favor science that much and, even when he was “a part of” the Aurora society, he was pretty much working alone on his bizarre doll project. 

Finally, there is a significant item to mention…

…because surely UT’s “treasure” has to be relevant and maybe even the primary reason he’s trying to bring back the dead. However, there is one interesting thing to notice: if we’re assuming that these lockets represent people UT was fond of, then Vincent and real!Ciel should be on this chain too and yet they’re not.

The explanation for this is rather simple

thanks to ch105

though, at least where Vincent is concerned:

So maybe it was the same for the twin:

UT didn’t think that his method could bring Vincent and the twin back as bizarre dolls (because the circumstances of their death damaged their bodies too much?), hence why he had no locket for either of them.

TL;DR if UT’s motus operandi in the Campania & Weston arcs is signficantly different from the mastermind’s motus operandi in this arc then it means…

2) Different “bizarre dolls”…

This part is assuming that Seb indeed never lied and that he was summoned through real!Ciel’s sacrifice (after a stab in the heart) and eating his soul, meaning that real!Ciel died at 10 years old and yet…

He definitely looks as if he never died and is going to turn 14 (?), like our!Ciel. 

Again, the difference in methods is probably the reason real!Ciel looks/acts so normal (compared to even Agares, UT’s last masterpiece) and all grown up, when it shouldn’t be possible since there aren’t many doubts that he died and Seb ate his soul, 4 years ago (unless Seb was somehow fooled?). 

Secondly, aside from real!Ciel, we probably only saw Lord Polaris so far (since Polaris is Sirius’ butler, real!Ciel is Lord Sirius and Agni’s killer used knives like Polaris)…

…and Agni, who was a very strong human himself, commented on his killer being unusually strong. So could Polaris’ abnormal strength be the result of these blood transfusions or, more generally, of how the blue sect brings the dead back? 

In the first place, only demons should naturally and abnormally be stronger than humans in Kuro!Verse (Shinigamis aren’t). However we know that, just like science is apparently currently used to bring the dead back, it was also used before in the story to introduce abnormal strength for humans…

…when Ciel met Finny while on another (older) mission for the Queen in Germany. 

TL;DR if UT is not the mastermind but someone else is: what if real!Ciel and the other star Lords came back as some sort of perfect equivalent to UT’s original bizarre dolls, because different scientific methods were put together to achieve such a “perfect” result? 

Of course, this only works if, instead of UT, the mastermind is…

3) …Thus a different mastermind?

At this point, there is only one possibility amongst the characters we already know, because there is only one person who showed interest in UT’s bizarre dolls…

…when Ciel told her about it, in June 1889, after the Weston arc.
Of course, Victoria most likely isn’t a supernatural being, so she was surely helped by her loyal John Brown who would do anything for her (remember that his eyes are always conveniently hidden => there is a good chance he could be supernatural).

The mastermind behind the sect being supernatural!Brown and following Victoria’s wishes works in regards to the whole timeline too:

• Bravat’s blue sect was originally used to experiment about blood transfusion until summer 1889…
• “if this method can be made to work, it will be a ‘miracle cure’ that can bring the mortally wounded back from the brink of death. I’ve heard that the army is actively conducting experiments to discover just what is necessary to make it a success” => Wolfram, ch114.
• …until Victoria and Brown learnt about UT’s bizarre doll from Ciel himsef in June 1889.
• Brown then started to experiment by himself, not knowing about UT’s own method, giving the “star Lords” to Bravat’s sect and switching its purpose to using blood transfusion (+???) in order to bring them back from death:
• that’s why the Sphere music hall was apparently opened during Summer 1889 and why the massive blood collects only started then
• + knowing about the incident at Weston would also be how Brown tricked the ex prefects and had Bravat using them to attract the masses.

Secondly, Othello said that the technology used by the sect was very advanced (and proof of supernatural involvement)…

and as @hitsugikuro pointed out before, it does look similar to Sieglinde’s own “lab” given to her by Queen Victoria and we know that war against Germany is the reason Victoria was so looking forward to Sieglinde working for “England’s future”.

War against Germany is probably also the reason Ciel was sent two years ago in Germany to take care of those researchers who were experimenting about artificially enhanced strength, which is when he met and saved Finny, by the way. 

We don’t know what happened then, but maybe our!Ciel brought back the formula for this “abnormal strength” to the Queen, or maybe England developed their own, which might explain why Agni described his killer (Polaris) as abnormally strong, if this technique was used on the “star Lords” as well.

Finally…

there might be a political aspect to the plan with the blue sect, which is to say that Bravat might have also gotten rid of those lords from the Parliament (ch125) so that it would be more difficult for the Parliament to refuse Victoria’s plans for war. Remember…

…the Queen might not rule but…

…she has some loyal helpers.

TL;DR the whole point to keep in mind is that Victoria is getting ready for war: that’s why she wanted Sieglinde and her SuLIN, why she sent Ciel to destroy that German lab where Finny was experimented on, why she wants zombie war weapons through bringing back the dead and why she might have assassinated parlimentarians who stood in her way. 

The blue sect is simply here to get her what she wants through first mastering the blood transfusion technique (to lose less soldiers), and then a way to experiment about bringing the dead back (to eventually turn more of them into war weapons). 

4) real!Ciel vs our!Ciel

Or why Brown and Victoria

would

bring back the twin now, in order to antagonize the current Watchdog. 
There again, there is a possibility for an answer, but first of all, let me insist on why this part

for now

also makes no sense if UT is the mastermind.

To this day, I do not think that real!Ciel’s actions are particularly predictable to anyone (please check this post by @akumadeenglish​). Seeing the latest chapters before the flashback, it’s evident that he’s not exactly in his right mind and there are many possibilities as to why that is, but for now he’s definitely supposed to be antagonistic. 

Meanwhile, UT is literally one of the few characters who has no reason to wish harm on our!Ciel: it always was noticeable in the way he used to give him advice, to watch over him, to imply that he wouldn’t let Seb have his way with his soul, because of UT’s long attachment to the Phantomhive family.

And since UT probably feels/felt the same attachment towards the twin (and doesn’t favor him, by his own words), it doesn’t make a lot of sense to think that he was hoping for real!Ciel and our!Ciel to fight against each other, or for our!Ciel to be isolated from anyone but Sebastian (since the sect took Lizzie away and real!Ciel almost took Soma away).

Back to the Queen and Brown though, they’re the ones who actually have a few reasons to be crossed with our!Ciel, since:

• After the curry arc, he more or less sheltered Agni (one of the culprits) despite his crimes 

• During the circus arc, he burnt those children, which led to him lying to the Queen about his report

• but the Queen found out and organized a murder party as a punishment (@ thanks Grey)

• After the green witch arc, he made sure that Sieglinde wouldn’t give or produce for the Queen anything like the SuLIN gas

• but the Queen and Brown weren’t fooled, which led to Brown comparing his efficiency to Vincent’s, who is dead.

So for all we know, the current arc might partially be the result of Victoria seeing through his lies about the SuLIN gas in ch108 and being fed up with him for not being a very obedient doggy. Thus why…

real!Ciel came back as an absolutely similar copy of the Queen’s current watchdog (same age (?), same height,

no change of identity, same earrings), so that she would replace the bad doggy with a more obedient one (who might be dependent on those blood transfusions and thus more loyal to her), without having to explain what happened to the previous one since the twins are now exactly similar. 

And if somehow you think that it makes her heartless to treat someone who works so hard for her that way, well, maybe it’s time to consider the idea that she has something to do with happened to Vincent and Rachel (and maybe Claudia too, who knows…). xD

TL;DR UT would have no reason to create a feud amongst his beloved Phantomhive family, considering he’s always been more of an ally to them than anything else, which leads to the final part of the explanation…

5) UT in ch108 & ch120

Or, if this theory is accurate, the red herrings that Yana showed to make us think UT was initially behind it all for the third time, since UT was with the twin in these two chapters. However, first please consider the settings of both meetings:

• it’s at night
• UT is creeping around, as if he wants to be undetected.

Then onto what supposedly happened…

I’ll quote @thedarkestcrow​‘s words once again to explain (A+ writing <3):

Instead of being the mastermind of the sect and supporting it I could rather see him trying to prevent or delay the sect’s success. The one time we saw him he gave one of the Ciels (and it was probably the twin) some liquid and remember his words? (see above)

When he visited the twin/Sirius that night the twin’s condition seemed quite well. Afterwards, however, we saw him having these arms (see above).

So UT may have intentionally worsened the twin’s state. Not with the intention to kill him, of course, but he may have tried to delay his full awakening in order to get time to maybe solve this situation in order to help both twins, not just one.

An idea which would in turn explain why he was checking the twin’s wrist (as if looking for any needle mark, to assess what was going on, maybe?) …

…as well as that single panel from ch120 that showed him as very affectionate.

Because remember what I said about UT’s “treasure”? If real!Ciel and Vincent didn’t have a locket, it might be because UT’s own method of bringing back the dead (through editing the cinematic records) didn’t make it possible for them to come back (too damaged bodies?).
So seeing the twin “alive” again must have felt like a shock to UT who cares a lot about the P family (hence the affectionate gesture of ch120), even if he probably tampered with the twin’s “well being” anyway… 

• because he wanted to stall for time
• because he couldn’t bear the idea that the twin too was used as another of the Queen’s pawns (since he dislikes her a lot).

6) Additional links

This part is just an addendum for me to add several links to posts that belong with this theory but that I didn’t particularly include in the explanation (otherwise I would have died):

• A reason for the slow pace of this arc by @thedarkestcrow (it works with UT or with Brown)
• why didn’t Tanaka look surprised about the twin coming back? 
• The Undertaker playing chess against the Queen by @hitsugikuro
• About Sebastian not being the Undertaker’s main enemy by @hitsugikuro​
• About the Undertaker trying to thwart the Queen’s war plans
• My personal interpretation of the Undertaker’s words in ch105 (to explain why he’s probably not after harming our!Ciel)

Please feel free to ask if anything is unclear, I’m so sorry that this was so long to read but I really believe that, if this arc is taking so long to be finished, it’s because it ties with older arcs/different subplots (besides the 2CT that’s taking a large place of the arc, obviously), hence the gigantic theory. xD

Thanks for reading so far. ^3^