Necrotic energy is the merging of soul energy and unholy energy. It is used in life draining attacks like the aura of spirits surrounding death giants, a wizard's death spell, and other forms of necromancy.
Maen Grirngrim was the first to put this type of energy to use. He created the spells Consumptive Field from Healing Circle, Death Knell from Death Ward, and Slay Living from Raise Dead, changing some of the parameters of these spells, replacing the life energy elements with those of necrotic energy.
Many schools of necromancy include both the study of necrotic and negative energy.