Soulfire

Harnessing the very powers of creation, you can supercharge your spells. Unlike Hellfire, Soulfire does not increase the damage of your spells. It makes your spells harder to resist, dodge, shake off. You can also try to experiment with your spells, think outside the box. 

Soulfire Ability: If a creature has advantage on magic saving throws, it does not have advantage when Soulfire is used. 

Ignore Resistance: For one charges of Soulfire, you can have your spell choose to ignore one resistance.

Increase Spell DC: For one charge of Soulfire, you increase your spell save DC by +2. For two charges, increase your spell save DC by +5. For three charges, increase your spell save DC by +10. 

Wild Magic: You want an extra effect with a normal spell, some extra oomph, something weird. Cost to be determined by DM. 

However, this does come at a cost. You give up a little bit of energy that is your soul each time you use Soulfire. This comes from a conversation you and Bob had.

Adran: So what happens if I use too much Soulfire, what happens to me?

Bob: What's five minus five Adran?

Adran: Zero.

Bob: I'm sure you'll get to the right conclusion eventually boss.

One charge of Soulfire reduces your max HP by 15%, rounded up to the nearest whole number. Subsequent uses reduce it by 15% calculated at max HP before any Soulfire has been used. The only way that it can be replaced is with time. Each long rest increases your max HP by 5. Each short rest increases your max HP by 2. You can only increase your max HP with a short rest once per day.