It's Hollow Knight as BeanisBrain said, and it is the original game that Silksong is a sequel to. Here's my little mini essay to answer your question:

It was a little indie metroidvania released in 2017 that fairly quickly ballooned in popularity, building up a surprisingly large fan base, becoming something of an indie sensation. Hollow Knight is now widely considered to be like the quintessential modern metroidvania, the name that always comes up whenever the genre is discussed. Even people who don't really play MVs tend to know about Hollow Knight. Personally I think it's a little overhyped myself and feel that there are other MVs just as good, on occasion even better, but it is undeniably a fun, addictive, and well-made game, with good reason for being considered a new classic.

The studio that created it announced that they were making a sequel to the game in 2019, naming it Silksong and saying they had intended it to be a DLC at first but that as it grew they decided it needed to be a standalone sequel. They included some impressive gameplay footage in that 2019 announcement, which made it seem like the release wouldn't be too far off. Because Hollow Knight had become such a massive viral success, the hype for Silksong was unsurprisingly immense. But as the years went by with no word from the studio about progress, a lot of fans started getting frustrated and there were jokes and rumors that it wasn't real, that it would never be released, that it was the new Half Life 3 (if you know about that saga). The "Silksong when?!" memes and vocal anticipation became almost as legendary as the original game.

Then just back in August, the studio put out the release trailer and gave a September 4th release date and people went nuts. A bunch of other games that had been slated for a September launch were postponed, their studios openly stating that they did not want to compete with Silksong and that magnitude of hype. The hour that Silksong released, Steam along with all of the console stores crashed from too many people trying to download the game. But now everyone who tried to download it upon release has it. And so e'erbody be playin' it. Those who aren't playing it, are playing the original, either as a replay to refresh their memories and prepare for playing the sequel, or for the first time, so as not to miss out or to learn why it's the sensation that it is.