Negative energy

Negative energy was a type of energy originating on the Negative Energy Plane. It manifested as a black, crackling surge of power. It was the antithesis of positive energy, being the "lifeforce" of undead creatures, healing them while causing harm to the living.

Clerics
Clerics were adept at using the power of negative energy. It was the power of negative energy that evil clerics channeled to awe or command undead, and it was used in the inflict series of spells (e.g., inflict light wounds) to injure living beings and heal undead.

Shadow Magic
As many people thought of positive energy as "light" and negative energy as "dark", it was popularly believed that negative energy was associated with shadow magic, and that they somehow originated from the same source. However, this was to misunderstand the key philosophy of shadow magic: the manipulation of the symbolic (shadow) rather than the real (negative energy). Even learned sages and some wielders of shadow magic made this misconception.

In fact, they were distinct and separate cosmic sources, though they could be used to accomplish similar effects. A shadowcaster who drained the life from a foe instead channeled it into the Plane of Shadow and summoned shadow in its place; they did not manipulate negative energy or the Negative Energy Plane. Both negative energy and shadow magic attracted many of the same entities and users, however.

Applications
Negative energy could be used to screen and invert the nature of positive energy and other negative energy effects. The spell shroud of undeath cloaked a caster in invisible negative energy, such they were effectively undead for all intents and purposes. Mindless undead perceived them as fellow undead, and inflict spells cast upon them were transformed into cure spells, and vice versa. Furthermore, a caster shrouded in this way could be stunned and panicked when a cleric channeled positive or negative energy to repel, rebuke, command or destroy undead.