I should clarify that as of now, the only hidden boss and level are in Hell Mode, and they are non-Castlevania related. Being able to access this level also requires you to take a specific path.
Being that I have been so focused on obtaining this particular achievement , I have not yet attempted to take the Castlevania route , so I have no idea if anything new is there in Hell mode…
The good news is that I’ve gotten good enough to make it to the hidden level and last boss in Hell mode…
The bad is that the last boss himself is a prick and continually heals himself while dealing crazy damage…