First of all thanks for playing the game :)
The game is a metroidvania so you have to go back in the previous rooms and do something with the pentagrams. I can't spoil anything here, but I wrote a devlog for this game and at the bottom of that devlog I wrote a complete walkthrough of how to 100% the game.
And my bad on not explaining things clearly in the game, because I wanted the players to find them for themselves.