A spell's essence describes its makeup, purpose, and means of operation. It's the very fabric of the magic that is enacted by the spell. Every spell falls into one of the following essences.
A spell with the compulsory essence forces intelligent creatures to act against their will. Compulsory spells use MP to change moods, allegiances, and behavior. This type of magic is often opposed by skills which keep a character collected: Guts, Mind Control, Sanity, or Virtue.
A spell with the dynamic essence alters or releases energy itself. Dynamic spells use MP to temporarily change energy into matter, invoke effects, and control power. Dynamic spells operate very specifically — even magic must obey the rules of the universe in which it operates. This type of magic is often opposed by Stamina or Guard. It's also very often aimed.
A spell with the mutative essence transforms matter. Mutative spells use MP to change materials into different types, tweak a thing's shape and size, or alter a thing's properties (e.g. buoyancy, flight, speed). This magic is often opposed by Resilience.
A spell with the illusory essence produces false sensory information. Illusory spells use MP to fabricate images, sounds, smells, tastes, and sensations in the minds of creatures or in the real world. This type of magic is often opposed by Lucidity.
A spell with the perceptive essence reveal information. Perceptive spells use MP to grant a creature enhanced or altered senses, or allow a creature to detect the normally undetectable. These spells are never harmful.
A spell with the stygian essence manipulates the border between life and death, pushing creatures past it or pulling them away from it. Stygian spells use MP to inflict pain, poison, disease, curses, blessings, and healing. This type of magic is opposed by Mettle or Vitality.
A spell with the transportive essence conveys objects or creatures through space. Transportive spells use MP to instantaneously relocate things across distances. These spells are not necessarily harmful, but creatures may not wish to be relocated, so this type of magic is opposed by Resilience.
A spell with the warding essence prevents or protects. Warding spells use MP to lessen effects or block them entirely. These spells are never harmful.