Holy is one of the least common elemental forces you will encounter as you explore the Dungeon, but also one of the most powerful in the late game. Players who do not worship the Shining One or Zin will have almost no access to holy powers, and there are only a handful of rare monsters with innate holy abilities.
Holy damage is the manifestation of the wrath of the Shining One or other good beings. Because of this, holy damage is usually only inflicted against targets that offend good beings. This includes all undead, demonic, and Necromancy-using monsters. Demonspawn or undead (including lichform) characters are also affected by it, but other species are exempt, even if they use Necromancy. Except for AC and deep dwarf damage shaving, there is no way for anything susceptible to holy damage to resist or reduce the damage they take from holy attacks. Some holy attacks also cause additional effects.
The holy wrath brand, while associated with TSO and only put on weapons that are themselves considered holy (meaning that demonspawn and undead cannot wield them), does not in fact deal holy damage. Rather, it causes your attacks to deal much more damage than normal to undead and demonic foes (deals 175% of normal damage on average). While these monsters are also vulnerable to holy damage, there are many other "evil" spellcasters and priests that are vulnerable to holy damage, but not to holy wrath weapons.
Very few sources of holy damage are available to the player. The only source available to players who do not worship a good god is the scroll of holy word. It lets you slow, fear, and damage (3d15 + 1d10, or more if you worship a good god) all vulnerable monsters in your line of sight. While demonspawn and undead characters are able to read these scrolls, doing so causes them to immediately lose half of their current HP.
There are also two gods you can worship to gain access to holy damage abilities. Followers of Zin can use his Recite, Imprison, and Sanctuary abilities to duplicate the effect of a scroll of holy word, while followers of the Shining One receive powerful Cleansing Flames, the ability to summon holy allies to your side, and eventually get to bless a weapon, giving it the holy wrath brand and possibly boosting its base stats.
Although monsters may occasionally wield holy wrath weapons themselves, most monsters have no access to holy damage. However, there are several holy monsters that can be found in the Abyss, Pandemonium, and certain Ziggurat floors which have very powerful holy attacks and special abilities. These monsters are a major threat to any holy vulnerable characters, so be extremely careful when facing them.
Players acquire holy resistance by being in good standing with one of the good gods. In addition, worshipers of good gods are occasionally acknowledged as allies by holy monsters, and they will not attack you at all. Also, note that only followers of evil gods take 100% of the damage from a beam with holy damage type. Most characters only take 50% of the damage. Only holy monsters have holy resistance, rendering them completely immune to the one player-available holy attack that can damage non-holy vulnerable monsters: the Shining One's Cleansing Flame.
Holy vulnerability renders you susceptible to many holy attacks you would otherwise be immune to, and greatly increases the damage you take from others (150% from beams; a random amount between +0% and +150% from holy wrath; +75% for holy flavored melee attacks). You can acquire it by:
- Playing an undead or demonspawn character.
- Being under the effect of the Necromutation spell.
Way back in the days of yore (namely, pre-DCSS), holy spells existed and were player-castable. These included such now verboten effects as Minor/Major Healing, Restore Strength, and Purify. The only one which remains is Cure Poison, which was originally a holy spell. Interestingly, the holy spell school had no miscasts, so unlike with Divinations they do not still exist as vestiges.
The school survived for monsters considerably longer, although its spells were gradually migrated away from it (as spells such as Healing needed to be used by evil monsters as well as good). The remains of the school were ultimately completely removed around 0.10. While some monsters have spells that are themselves considered to be holy, there no longer exists a defined school of holy spells.