Yacht Club Games has announced a firm release date for the Shovel Knight's Plague of Shadows expansion. Confirmed for September 17 (ie, next week), Plague of Shadows replaces the game's nominal Shovel Knight with the Plague Knight – one of the antagonists in the vanilla game. That means you'll get explosives, bomb jumps and alchemy, as well as a new crafting system.
Best of all, the expansion is free. In addition to new routes through existing levels there are also some new areas, new bosses and a new Challenge Mode. The latter includes speed run challenges and boss encounters, among other things.
This trailer was released last month, but gives a good overview of what to expect: