Filming for the fourth season of "Sherlock" has already wrapped as co-creator Mark Gatiss teased on his Twitter account. Fans should get ready for a darker season as consulting detective Sherlock Holmes and his best friend John Watson return to 221B Baker Street.

Gatiss, who also serves as a writer on the show he co-created, shared a snap on his Twitter handle of a letter with Sherlock's name and address scribbled onto it. It is pinned down by a swiss army knife. Gatiss, who also plays Mycroft Holmes on the series, captioned the photo: "The Game is over...for now."

And of course, a dark season calls for a dark villain. After an intense battle against Moriarty (Andrew Scott) and killing Charles Augustus Magnussen (Lars Mikkelsen), Sherlock now has to face a dreadful new villain played by Toby Jones. BBC announced the casting in May earlier this year. 

"He's completely different. It's a completely different character. He's the darkest villain we've had," co-creator Steven Moffat teased to Collider. "This guy is the purest evil. Sherlock is actually appalled by him. He's the most evil villain we've had. I don't think that when you see it, you will disagree. He's horrific."

The teaser trailer certainly backs up Moffat's words of Jones being the most terrible villain. It was previously reported that the clip featured scenes nothing short of action-packed, with explosions and guns all around. 

Season 3 ended with Moriarty appearing on television screens, taunting the world with his words: "Did you miss me?" The Christmas special, titled "The Abominable Bride," saw Sherlock retreating to his mind palace and concluding that Moriarty is definitely dead.

As for the possibility of future seasons, nothing has been confirmed. However, Moffat and Gatiss have both expressed their desire to continue the BBC series, but not if it means the quality will suffer. Benedict Cumberbatch, who plays the titular detective, is also unsure of the future. 

"Sherlock" season 4 will premiere in 2017.