enerally, for a format like Legacy, the best decks tend to sit in that upper sphere of viability for quite a while. The format has no rotation, so without something vital being banned or unbanned, the format's best decks have the luxury of staying there. Of course, new sets do have an impact on the format, but you are talking about increasing the card pool by maybe 1%. Compare this to Standard, where Return to Ravnica makes up more than 20% of the format. So, while new lists are always being invented, those lists also have to do well to really make it into the spotlight. Shawn Riggin did just that with his take on Mono-Black Smallpox.
So, as any seasoned player knows, most symmetrical effects can be taken advantage of, turning any potential negative into a positive. Shawn recognized that as well, because this deck has a whole lot of those! Innocent Blood, Smallpox, and Nether Void are supposed to hit both players, but not when you don't have a creature in play or when you have a Crucible of Worlds on the battlefield! The activation on Liliana of the Veil is another symmetrical effect, but this deck gets to back it up with so much more discard that even that tends to be unfair for the opponent.
You might be looking for the win condition right now and find yourself a bit confused. The deck plays no creatures, has a Planeswalker that can't literally win the game, and contains a bunch of disruptive instants and sorceries.How does it win? Well, like most prison decks, the road need not be quick, as the deck is so good at denying the opponent resources you get to win on your own terms. In this case, a pair of Cursed Scrolls and a playset of Mishra's Factory should do the trick! It isn't fast, but it does the trick!
Shawn Riggin's Mono-Black Pox
Legacy – Top 16, StarCityGames Open, St. Louis