Holy is the power of the Light also known as Creation. It preforms attacks as well as blessings and healings. Holy has been the main religion through the ages in many cultures. The Wysterians saw it as the One God with the Light, the Phairans worship the actors of holy power known as the Heavenly Host and an elemental god of Light even exists as an extension of this power.
The Solarians brought the knowledge of holy magic to the world. They taught the ancient races about it. When they left the planet they species they left behind slowly deviated from the traditional teaching and created their own variations of working with holy magic.
The common appearance of holy magic is the Heavenly Host. They are accepted and worshiped deities to the Aetherian people and are of major study in the Temple. There is a great deal of difference between the Temple's teachings and the Arcane Sanctum's. The library is split between the two lists of spells. The Ryuko had their own version of holy magic in their Shinto practices. Their monks, priests and mikos were able to preform holy spells that triumphed over the Entropic Dragon's corruption.
Evil is talked about a lot with Holy Magic. Evil is considered Unholy, Negative, Death and Entropy. What counts as those?
Asoth's Followers = Negative & Death
Dahhak = Negative
Demons = Unholy
Mouko's People = Entropy
Reavers = Death
Witchcraft, Malicious = Negative
Holy cannot heal the corruption of Entropy as it is technically not an evil magic itself but it can be viewed as unholy. Holy's only counter to Entropy is to heal damage since it is counter the energy of chaos and deconstruction with order and creation.