So, let’s chat about Eurus and Moriarty
There have been a number of metas suggesting that Eurus killed Redbeard, and that she was identified as a sociopath early on and institutionalized. Personally, I don’t think that killing a dog would have been enough to get her institutionalized by itself, so I think that she probably also attempted to kill Sherlock. Again, there have been other metas talking about Sussex Vampire, in which a child attempts to poison their half brother and poisons the family dog. I think it’s likely that Mofiss is going this route for The Final Problem because Eurus refers to herself as a vampire when texting John, and also because the trailer contains a visual reference to the 1992 Granada The Last Vampyre, which is another Sussex Vampire adaptation. So, Eurus tried to kill Sherlock and was sent away, and Sherlock doesn’t remember what happened and has fused the memories of the loss of his sister with those of the loss of his dog.
So, Eurus is locked away, and at some point, maybe she has the possibility to get out, but Mycroft makes sure she stays there. And she’s seething with rage but powerless to do anything about it. Until suddenly her world changes.
Eurus told Sherlock that her entire world turned on one name, and I’m pretty sure that name was:
(Seriously, Sherlock, Napoleon? Elvis? wtf)
So, how did Eurus come to hear that name? I’m guessing that he found her, and I think it’s because he heard her name first.
There are three clues Moftiss have given us which I think explain the Moriarty/Eurus connection:
Mycroft admits to John that he traded bits of information about Sherlock’s past while he was interrogating him in order to get Jim to talk.
Mycroft tells Sherlock, “you may consider [Magnussen] under my protection,” to which Sherlock retorts “I consider you under his thumb” in HLV.
Magnussen thinks “Redbeard” is Sherlock’s biggest pressure point.
Now here’s what I think happened:
I think that when Mycroft was “interrogating” Jim (and I say this in parentheses because IMO it’s obvious that it was really Jim interrogating Mycroft) he maybe let it slip that their sociopath sister Eurus killed their family dog when they were kids (I doubt he said that explicitly, but this is Jim we’re talking about; he read between the lines).
I think that Jim shared this information with his blackmailer friend Magnussen, who used it to keep Mycroft in check while he went after Lady Smallwood.
And then Jim let Eurus loose as his final act of mayhem. Another nifty meta I’ve seen is that Eurus, one of the four seasons, is associated with Autumn. “I owe you a fall” takes on a rather different meaning in this context.
They are natural allies, IMO. Jim wanted to burn out Sherlock’s heart. Eurus wants to burn out Mycroft’s. As Magnussen already pointed out: John is Sherlock’s pressure point, Sherlock is Mycroft’s. Knock one over and watch them fall like dominos. Also it’s clear that Eurus has a lot of information on Sherlock, and she had to have gotten it from somewhere. Methinks a little spider told her all about her brother.
I think it’s Eurus who was responsible for putting Jim Moriarty’s face all over London with the “Miss Me” message. It’s foreshadowed in TAB that the new “Moriarty” is a woman. Also, as @rottenbrainstuff pointed out, “miss me” also makes a lot of sense coming from Eurus, who hasn’t seen Sherlock in decades. No babe, he doesn’t miss you. He doesn’t even remember you. But he will.
So, Eurus pretended to be Moriarty because she wanted Sherlock to come back from exile. Because what fun would it have been for Sherlock to die in Eastern Europe? She wants to kill him. Or possibly, she wants to make him kill Mycroft (those terrifying metas that propose that scene with Sherlock, John and Mycroft together in the bunker is a scene in which Sherlock is forced to choose between killing either John or Mycroft comes to mind).
So yeah, Moriarty and Eurus have been working together since Season Two, IMO. I think that The Final Problem is going to take us back in time, and give us some flashbacks of Jim and Eurus setting their plan in motion. And Sir Edwin, who made that footage of Sherlock go away so that he didn’t need a pardon? I think he’s codename Porlock, as in, Moriarty’s agent in Valley of Fear. He’s been working with them, too.
