Echoes of the Void

Wow, that's an interesting spoiler. How can we compare the strength of this spell (how much it will drain)? (e.g., compared to Spell Power, Master of Darkness, or something else... what's the arithmetic like? In other words, is it worth taking Darkness to enhance it?) Because I've been leveling up Twilight, with red griffins + readiness + vampirism.

I noticed that when performing the ritual to banish demons from a peasant hut, it doesn't always deduct 5 crystals. I've seen it deduct them, and then not deduct them if I replay. I thought it was glitching, but I checked.

Without skill: 10% + spell power * 2%
Basic: 20% + spell power * 2%
Advanced: 30% + spell power * 2%
Master: 50% + spell power * 2%