This article's content is accurate to the most recent public game version, 0.4.8d.
"Deals fire damage to all enemies. Really not the kind of magic you where expecting from Natalie."
Cost: 8 MP
Source: Natalie - Unlocked at the world map after defeating two Magicos.
Abyssal Inferno is a spell that deals moderate 
Fire damage to all enemies.
Although pricey, this is an excellent offensive option, and very MP-efficient when dealing with more than two opponents.
The following enemies can cast Abyssal Inferno: