Downfall - A Slay the Spire Fan Expansion (the full name), is a bit of a mouthful but it's also spilling over the side with features. So much so, that it got a Steam release outside the workshop — you still need to own Slay the Spire though of course as it's a big mod.
Slay the Spire is still easily one of the best deck-builders around, and now with this free fan expansion out, it's even better. You get access to a full new character for the Standard game mode, and then for the new Downfall game mode there's a bunch of Villain characters. Currently there's seven of them to pick from, with more on the way. Gameplay is flipped too, with you going down the Spire instead of up. Finally taking the fight to the heroes.
There's a lot to this expansion includes new Relics, Potions, special 'Boss' cards, a large collection of Events, Curses, Daily Run Modifiers and the list goes on. It's absolutely huge and definitely worth a go.