Spontaneous Casting is a game mechanic introduced in Dungeons & Dragons 3rd Edition as a mainstream alternative to the traditional Dungeons & Dragons mechanic of Vancian Casting. It can be most simply summarized as a cross between Vancian Casting and a mana system.
To go into detail, Vancian Casting requires a caster to individually memorize each spell they want to cast for a day. Once expended, those spells are gone until they can be memorized again - it's like depleting a quiver of arrows. This means that if you can take away a wizard's spellbook, they lose all ability to regain any spells they cast until they get their hands on it again. In contrast, Spontaneous Casting means that the spellcaster simply "knows" their spells, and can use their spells-per-day mechanic (called "Spell Slots") to fuel whichever spell they want as they want it. And when they run out of spell slots? A simple nap, and they're ready to go again!
Spontaneous casters, unlike Vancian casters, generally can't learn spells from external spell-slots. This means that a sorcerer will never have the breadth of utility options that a wizard will have, because their total spells known is smaller. But, potentially, a spontaneous caster will have more "encounter versatility", since a vancian caster is completely shafted if they are not given time to prepare their spells in advance, whilst a spontaneous caster can immediately use whichever of their spells known is necessary for the situation at hand. Nearly all spontaneous casters cast off Charisma.
One variant of Spontaneous Casting originated with the Spirit Shaman class. Rather than known spells being set in stone when obtained, a Spirit Shaman picks his spells known each time he recovers spell slots. This variant would be used for Pathfinder's Arcanist and Vitalist, as well as many classes in 5th edition (see below).
This mechanic ultimately inspired the AEDU System of Dungeons & Dragons 4th Edition, and is the foundation of casting as a whole in Dungeons & Dragons 5th Edition; in 5e, the "Prepared Caster" classes have a Vancian-style ability to change their "spells known" during their down time, but maintain the Spontaneous caster's ability to cast whatever they want, so long as they have appropriate spells per day.
List of Spontaneous Caster Classes
- Arcanist: Uses a variant as mentioned above.
- Assassin: An unusual Intelligence based spontaneous caster that knows a few spells to help them be sneaky or make a clean getaway.
- Bard: Was changed from Vancian Casting to this in 3rd edition.
- Bloodrager: Barbarian with a bit of Sorcerer.
- Favored Soul: Spontaneous casting from the Cleric list. Had three fundamental flaws that crippled it 1: Gave them Multiple Ability Dependency as Nerf to full casters that Clerics didn't get 2: One of the important things for a Cleric to do is to remove aliments during off days (Fight has been cursed? Better suck it up till you go back to town because the Favored Soul can't fix it). 3: Lack of Turn Undead made qualifying for most divine prestige classes impossible.
- Hunter: Spontaneously casts spells from the Druid and Ranger lists. Wisdom based.
- Inquisitor: A spontaneous divine caster with its own list. Pairs it with some skill monkey ability. Wisdom based.
- Oracle: A Pathfinder creation to create a "core" divine magic full caster class using Spontaneous Casting. Fares better than Favored Soul did thanks to not being MAD, Mnemonic Vestments mitigating the aliment removal spells problem, and Pathfinder not using Prestige Classes much (and what were didn't need Turn Undead).
- Skald: An angrier more martial Bard.
- Sorcerer: The original Spontaneous Caster, and the class for whom the mechanic was invented.
- Spirit Shaman: Spontaneous casting from the Druid class.
- Summoner: Though spontaneous casting is largely secondary to the big beatstick they create.