Who is Eurus Holmes? The latest 'Sherlock' antagonist could be the biggest one yet

Sherlock has been teasing a Holmes sibling for a while, presumably, under the name "Sherrinford," which would intimate there's a third Holmes brother waiting to be revealed. But whether or not Sherrinford is a real character — be it a codename for someone else, and, perhaps, still another member of the Holmes lineage — we finally got confirmation that a third Holmes exists in "The Lying Detective." 

It's Eurus Holmes, ostensibly, Sherlock's long-lost sister that's been waiting in the wings under several disguises. In season four, she's posed as John Watson's flirtatious stranger on the bus, John's therapist and the daughter of serial killer Culverton Smith. The Eurus reveal sets the stage for the season four (and perhaps, series?) finale to pit Holmes against Holmes, with Sherlock going up against someone he probably didn't even know exists. 

But if Sherlock wasn't aware Eurus was his sister, where does that leave Mycroft? In the season four premiere, Mycroft says he's not "given to outbursts of brotherly compassion" because of what happened to the "other one" — was he talking about Eurus the whole time? 

Obviously, there's plenty of questions fans will have about Eurus ahead of the season four finale — but her importance to Sherlock's arc is unequivocal. At the center? Her admittedly strange name. 

Eurus' name is rooted in Greek mythology. 

In Greek mythology, Eurus (or Euros) is the Greek god of the east wind, a relatively innocuous clue if not for Sherlock's fixation on the term "East Wind." Sherlock has been referring to the east wind since season three, following James Moriarty's post-mortem message for the detective. 

"The East Wind takes us all in the end," Sherlock tells John in the season three finale, "His Last Vow." "It's a story my brother told me when we were kids. The East Wind — this terrifying force that lays waste to all in its path. It seeks out the unworthy and plucks them from the earth." 

Hmm, yes, so that's foreboding. If Mycroft was telling this story to his brother when they were kids, and Sherlock perceived the East Wind as symbolic (as one would!), it would suggest Mycroft was aware that Eurus could return to their lives in some form. Plus, considering Eurus' propensity for convincing disguises — much like Sherlock in both the series and the novels from Sir Arthur Conan Doyle — it could mean she's been watching Sherlock from afar for quite some time. 

Where does this leave Sherrinford? 

The Sherlock cast left a fun tease at San Diego Comic-Con by holding up signs which said "Thatcher," "Smith" and "Sherrinford." Now we know the first was a reference to the season four premiere and the busts of Margaret Thatcher that were being destroyed, while "Smith" refers to the aforementioned serial killer Smith in "The Lying Detective." But if Eurus' name is, well, Eurus, what exactly is Sherrinford? 

It's clearly something important, as Mycroft's notebook as seen in "The Lying Detective" has a number under the Sherrinford name. But if that's not the name of Holmes' secret sibling — as some fans had guessed — could it instead be a place? 

If Eurus has been hiding away this whole time, and Sherlock probably doesn't even know who she is, was she locked away at some type of institution? And could this institution be called Sherrinford? If Mycroft has a number saved under the name Sherrinford, perhaps he checks in every now and then to keep track of his sister's whereabouts. 

That's one theory, but alternatively, it could be another member of the Holmes family. That could prove a bit exhausting, except if the hypothetical fourth Holmes is being played by, of all people, Tom Hiddleston. There's been rumors abound that Hiddleston could be a third Holmes brother (Sherlock actor and producer Mark Gatiss has been stoking the fire with cheeky tweets, of course), and it's still not out of the question. That said, a hypothetical on-screen meeting between Hiddleston and Benedict Cumberbatch as two brooding Holmes might actually break the internet. 

In any case, the mysteries surrounding Eurus and Sherrinford will likely be answered in season four's finale, "The Final Problem." After all, it could end up doubling as a series finale for Sherlock, whose production has become increasingly difficult with the rising stardom of its leads. Here's to Sherlock ending with a bang, which, based on the teaser trailer for "The Final Problem," could literally happen. 

The Sherlock season four finale, "The Final Problem," airs Sunday at 7 p.m. Eastern on PBS.