The premiere of Sherlock season 4 featured the death of a major character, Mary Watson, and strained Sherlock and Watson's friendship. The show's co-creator and star Mark Gatiss insisted that the finale episode will be "more action-packed than we've done for a long time".

The three-episode series will air its finale, titled, The Final Problem, on 15 January. Gatiss told TV Line, "There is quite a lot of action" to be had in season 4. "Episode 3 is probably more action-packed than we've done for a long time, if ever. Definitely more fighting."

Sherlock also gets a formidable new adversary this season in Culverton Smith (played by Toby Smith), an infamous baddie from the original Arthur Conan Doyle stories. "He's a very 21st-century villain, and without hyperbole, one of the scariest things we've ever done," Gatiss said. "Toby is absolutely terrifying".

With a new villain emerging, fans are wondering if Moriarty will be back to haunt Sherlock. Although Moriarty was featured in the trailer, Gatiss insisted that he's still dead, and offered a little tease. "Quite how he is in the [season] is a very interesting question."

Previously, Gatiss, who also plays Sherlock's brother Mycroft Holmes, spoke to Vanity Fair about the new villain Culverton Smith, and how he stands in comparison to Moriarty. "The danger with anyone other than Moriarty is you run the risk of them appearing as a diluted version. Thus our other villains are very different: Magnussen was a businessman in the Murdoch vein — not evil as far as he's concerned. Just totally amoral. Culverton Smith is different again — you'll have to wait and see! — but very much a man of these strange, rootless, dark times. What can you not do if you have power?"

Sherlock season 4 airs on Sunday on BBC One in the UK and on PBS in the US.