The Barrowbrook Frontier
Positive and Negative Energy
To the people of Parthenos, positive and negative energy represent the ultimate creative and destructive sources of the universe. Positive energy is the Prime from which all springs, and negative energy is the Void, to which all returns.
Within the standard D&D alignment structure, this means that positive energy is inherently Good, and negative energy is inherently Evil. All spells that deal in positive energy have the [Good] descriptor, and all spells that deal in negative energy have the [Evil] descriptor.
Uncontrolled mindless undead such as skeletons and zombies are Chaotic Evil, and have both the [Chaos] and [Evil] subtypes. Such creatures, when uncontrolled, spend their every waking hour marauding across the countryside, attacking and destroying every source of life they can find. When controlled, mindless undead do exactly as they are ordered, but always choose to interpret their orders in the most malicious, destructive way possible.
Likewise, sentient undead are creatures of pure evil. They, too, are generally Chaotic Evil, and are motivated by the desire to destroy all that exists. Some rare undead might be Lawful Evil, but their psyches are always impelled towards destruction and malice at least as much as control and domination.
Clerics who are good-aligned may not cast inflict spells, as those spells now have the [Evil] descriptor. Likewise, clerics who are evil-aligned have no capacity to heal, as curative spells have the [Good] descriptor.
Neutral clerics have may choose whether they channel positive or negative energy, as normal, but actually making use of this ability inevitably impels them towards either good or evil. Neutral clerics are exceedingly rare, and those that do exist are those who are slavishly devoted to maintaining balance.