The Sublevels will be a new addition to Apeirophobia that is teased alongside the rest of Chapter 2's Finale which will complete the Level 24 finally. The Sublevels have been teased multiple times and the devs have become a lot more transparent with what they are working on as much as possible whle also still keeping somewhat of a mystery as to what is going to be available. They have so far only released a full trailer which seems like it will play when you enter the sublevels. (LINK TRAILER)