So I'm literally near the end of the game where you have to expel your curse to Belial's blob. Except, I can't use level 5 transform (Expel Curse). I've tried all sorts of things and ultimately used a power stone before dragging the demon to get to the last area only to find out upon looking it up that I'm missing the "mantle of the ancients". And now I'm far too far to go back, I have passed multiple points where there's no turning back and just locked here.
Anyone know a way to get around this? Heck, even an editor or something would be much appreciated.

Hey, You've probably left this game far behind now, but I just recently finished this game myself for the first time.
On loading a save just before the birth chamber, I was successfully able to back track to the main entry point of the Subterranean Cavern. There you must go close to each of the 4 colored crystals and cast level 1 spark to call the elevator down. This will get you back into the City of the Ancients where you must raise the tower to find the 'Mantle of the Ancients'

On the note of Ancient Magic Stones, If you cast prism level 2 (or likely use the cowards shield), you can avoid loosing stones to those glowing balls of light in Belial's Laboratory

I've run into my own troubles at the endgame. I have the Mantle of the Ancients, but when I try to expel my curse into Belial's blob, I get the same result I always get when casting level 5 transform: A random black blob slaps Luther in the face, taking away 75% of his HP. That's it. I've looked up walkthroughs and video let's plays, and I can't see that they're doing anything even slightly different from what I'm doing. Anyone have any ideas?

Alright, I managed to figure it out on my own. You can click on Belial's blob, and doing so leads Luther to make a comment the first four times you do it:

"Incredible! This is growing right inside the room!" [or something like that]
"This is the mighty Belial? Hah!"
"I expected something more threatening-looking."
"He looks ready to kill right now."

...however, clicking on Belial's blob even once makes Luther incapable of expelling his curse into it. You have to cast level 5 transform without doing anything else.

I wonder if this is a glitch, only because it seems counterintuitive that something like this would bring the game into an unwinnable state.