I have everything in the game except the heart block from the monolith. When I go to the monolith room there's nothing there.

Have you looked at the floor, I wonder if there's something in your inventory that looks the same?

@MattBosten said:

Have you looked at the floor, I wonder if there's something in your inventory that looks the same?

I think all you needed to ask was "Have you looked at the floor?"

I had heard some stuff about "The black monolith" on the internet but never really read into it.

Then when I did this puzzle today I expected an anti-cube, but when this happened I was like "oh no! ARGH! NOW I'M IN ON IT!" and got really scared lmao.