Magical Exhaustion is an effect a spellcaster suffers when they cast a spell that consumes more energy than they have available.
Any spell can be cast as long as the caster has at least one point of energy. However, a creature's energy can never be negative. If a spell costs more energy than its caster currently has, the caster's energy is depleted to zero and they suffer Necrotic damage equal to the difference. Necrotic damage ignores all armor and resistances.
Magical exhaustion damage taken from casting an enchantment counts as injury damage. Magical exhaustion from any other type of spell counts as normal necrotic damage.