Just posting about this because it tripped me up - at the end of the game during the Crimson Tower section you need 3 glass spheres (Flame sphere if identified). There is a niche that transforms any item you place inside of it into glass spheres, but if you don't take 3 with you and pass by the into the void you can't go back.

If you are using All-Seeing Eye you can just teleport yourself back to get some, otherwise I hope you still have a save prior to entering the void portals.