Not all spells in Dungeons & Dragons require the use of slots to cast, as there are some that can be completed as part of an arcane ritual. The benefit of a ritual is that the caster only needs to have the spell prepared or for it to be in their spellbook (if they're a Wizard) in order to cast it. The downside is that the ritual takes ten minutes to cast, so the spell can't be used in a hurry. Ritual spells also cannot have their level increased in order to make their effect stronger.
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