9th. [...] The higher a spell's level, the higher level a spellcaster must be to use that spell". Spell slots Spellcasters can only cast a spell they know or have prepared if they have an available spell slot. This mechanic originated out of the Vancian magic system where "the number of memorized spells is strictly limited by the magician's memory capacity in proportion to the spells' difficulty levels, effectively granting a number of spell slots". Dungeons & Dragons simplified Vance's formula "to a number of spell slots scaling with the player character's level". In the 3rd edition, the mechanic to cast a spell was described as a drain on a spellcaster's resources which reduces their capacity to cast additional spells. The 5th edition Player's Handbook (2014) states that "regardless of how many spells a caster knows or prepares, he or she can cast only a limited number of spells before resting. Manipulating the fabric of magic and channeling its energy into even a simple spell is physically and mentally taxing, and higher-level spells are even more so. [...] When a character casts a spell, he or she expends a slot of that spell's level or higher, effectively 'filling' a slot with the spell. [...] Finishing a long rest restores any expended spell slots". Ritual magic The 2nd edition sourcebook Tome of Magic (1991) introduced many new variant spell concepts. The cleric class gained access to community-powered spells, cooperatively cast spells, and "super-powerful spells that required a quest before they could be cast". Most of these new concepts have disappeared since 2nd edition, however, "a few of the quest spells did show up in [3rd edition] as 9th-level priest spells". The 3.5 edition sourcebook Unearthed Arcana (2004) added optional rules to Dungeons & Dragons including variant systems for magic. It expanded on the concept of incantations which were introduced in the d20 System Reference Document as part of the variant magic rules. Incantations were designed to give non-spellcasters access to magic and to give lower level games access to higher level magic. Unearthed Arcana states "at its simplest, casting an incantation is akin to preparing and cooking something according to a recipe". However, there are drawbacks to attempting to cast incantations: they are time-consuming, they require particular circumstances and sometimes multiple casters, the components can be expensive, and there's no guarantee the incantation will work successively. In 4th edition, rituals became