Re: Sherlock
The new villain is likely a modern update of Sebastian Moran. In the classic stories, he's an ex-military man (making him a foil to John), who has since used his talents to become a master assassin. He worked under Moriarty before he was killed in his confrontation with Holmes, and is now out for revenge because of it. He's known as the second most dangerous man in all of London (Moriarty, of course, was the first), and he also made his first appearance in "The Adventure of the Empty House," which this episode was based on.
But yeah, amazing episode as always. I love the way they tease how he did it and still make you wonder if the version he confessed to is true or not.
Thinking about it again, it's also possible that it could be Charles Milverton, since he was known for having a collection of random personal artifacts that he took from people. And the villain here definitely had a collection of random stuff in his room.