New Bosses, a new storyline, and a new game mode. Whether that's a boss rush or something original we do not know. But we do know that this last update isn't simply the last one for free but in perpetuity. This was my favorite new game last year and while the switch port still languishes in QA testing for the foreseeable future I will pester everyone I know (that games) to give it a shot.
Please play Hollow Knight, Ya'll. It's the best $15 I've spent all year.