So I'll start with my deduction, and then offer the explanation afterwards.

Ready?

Undertaker is Vincent Phanotmhive's father and Ciel Phantomhive's grandfather.

Lemme read your mind real quick… yeah, I need to explain that one, don't I? no problem. Right this way.

I'll start with the scene from the Green Witch Arc in which Undertaker visits Diedrich. He cries over the image of Vincent. Now why would a rogue shinigami cry over a mortal? Because he isn't just any normal mortal.

Next let me remind you of the necklace the Undertaker lost to Ciel on the sinking ship. It carried the lock of Claudia Phantomhive, Vincent's mother. Now why would he have that if not because she had meant a lot to him? There is definitely a connection between Undertaker and the Phantomhive family.

Now back tot he german shinigami in the green witch arc. At first thy wonder how Ciel can see them so readily, however after checking his family tree they are no longer surprised. Yes, they also mention that due tot he pact with a demon Ciel can be considered half dead, but they do point out the family tree in addition tot hat. And in the picture in the manga we see Claudia Phantomhive, mother of Vincent Phantomhive but the father of Vincent is obscured. And while we're at it… since when is the name oft he mother kept after marriage? Not in this time they weren't…. maybe because they weren't married? (oh scandalous… XD)

Now those were the big, blatantly obvious hints, but there are many smaller ones that I'll list now.

· Undertaker went rogue about 50 years ago which fits with the timeline

· Sebastian points out that Undertaker wasn't out to harm Ciel when the ship is sinking (who are we to doubt Sebastian of all people….)

· This grief could also explain why Undertaker demands laughter as payment

Now if that is all true, then why…. Why in the world did Undertaker feed his grandson a vial of some liquid in his sleep?

This is where part two oft he theory starts. Now I admit, it's not as solid as part one. But if you're still interested, keep reading

I think we will get the anime ending in the manga

For those that are manga readers only, the anime ends with a huge mess of events that lead to Ciel losing his soul entirely (or whatever the hell that was… weird is the word I would use…), becoming a demon himself and Sebastian being his eternal servant (also, the dubbing is outstanding, so go watch).

Only I think it will be through Vincent.

Undertaker kindly provided the information that the Earl of Phantomhive is still around. Now… he wasn't being terribly specific, perhaps intentionally to confuse everyone or maybe because both earls would soon be one anyways?

Undertaker is also experimenting with walking corpses, he clearly can't accept death as the end. Why? He's immortal, so clearly it's because of someone else.

He mentions there wasn't a body left of Vincent to bury. That can mean one oft wo things. Either Undertaker snatched the body (so he only wasn't buried), or he was really burnt to a crisp. And in any way, it means he cared enough to check. Else he wouldn't have this bit of information in the first place.

I believe the Undertaker needs a vessel to get his son back. Ciel and Vincent look both alike, same gender and share 50% of DNA. It doesn't get much better as a vessel, doesn't it? Okay, aside from twin brothers, but we don't have that…

His biggest obstacle in this is Sebastian of course, which makes this plot viable in a manga. It also is something Sebastian couldn't just smite with a swipe of his hand and from a story telling point of view things like those are important too.

Now one big thing we also have to look at: motif.

Because aside from the obvious reason of wanting his son back, there is more to it. Redemption. If you were Vincent and had an immortal rogue shinigami as a dad, how in the world do you die like this in the first place?! Well, maybe Undertaker screwed up and is now trying to make up for his mistake.

So… here we are. Two souls in one body, but would Vincent even take over? I think this is a great opportunity for a plot twist. Because if he had a choice, I think Vincent would refuse. He honestly loved his son after all and wouldn't want to do this to him.

So… now the things friends addressed when I discussed it with them:

Honestly, neither me nor the two friends I discussed this with can fault part one oft he theory, the family connection. They were dead convinced after i brought it up with each. But feel free to if you can think of something that contradicts it.

Why would a grandfather hurt his own grandchild like that? A friend brought that up when I discussed it with him but I was just laughing. Because let's face it… Ciel is a brat. I love him, yes, but I can't really hold it against anyone if they don't. Vincent is far more lovable than Ciel XD

Wouldn't Undertaker try to take out Sebastian all the time?! True, Sebastian is in Undertaker's way. And he injured Sebastian quite a lot on the sinking ship. Even Ciel knew they had cut it close this time. And really, is there something Sebastian can do at this point? Seeing how Ciel was already force-fed the content of this vial? So… again when a friend brought it up, it was more like a confirmation for me personally.

After all that, I am convinced when it comes tot he family tree. I am not entirely sure about the whole soul merging part myself. It would make for a good story and it would redeem the mess that the second season of the anime was (and from what I heard there are so many flames out there and I too scratched my head). But I wanted to present both theories linked like that instead of picking them apart.

Hope you had fun :)