Casting an Incantation
At its core, casting an incantation means having the required incantation components, then succeeding at a number of Knowledge (arcane lore) checks during the incantation’s casting time. Each incantation lists how many Knowledge (arcane lore) checks are required to cast the incantation successfully.
Unless otherwise specified, the caster makes Knowledge (arcane lore) checks every 10 minutes. Failing a Knowledge (arcane lore) check doesn’t mean that the entire incantation is a failure, just that the previous 10 minutes have been wasted. However, if you fail two Knowledge (arcane lore) checks in a row, the incantation immediately fails. The consequences for failure are detailed in the description of the specific incantation. Even if the incantation fails, material components and experience points are still lost and the backlash still takes effect.
Most incantations require components not unlike spells, including verbal, somatic, focus, and material components. In addition, some require secondary casters (abbreviated SC), cause some sort of backlash (abbreviated B), or cost the caster some experience points (abbreviated XP).
Some incantations require multiple participants to cast successfully. These secondary casters (abbreviated SC) are indispensable to the success of the spell. No matter how many people are gathered in the dark room, chanting with candles, only one character—generally the one with the highest Knowledge (arcane lore) check—is the primary caster who’ll make the relevant checks. Secondary casters can’t help the primary caster with the Aid Another rules, but their presence is required for certain aspects of the ritual nonetheless. If an incantation requires some other skill check, any of the secondary casters can make that check if they have a higher bonus than the primary caster. Even if you’re not a required caster of the spell, you can step in and make the non-Knowledge check if you’re better at the relevant skill than the actual caster.
Some spells damage or drain the caster in some way. They have a backlash component, generally damage, negative levels, or some other condition. The caster takes the backlash regardless of the success or failure of the spell.
Saves and Spell Resistance
If the incantation allows a save, the formula to calculate it is included in the spell’s description. For checks to overcome spell resistance, divide the incantation’s Knowledge (arcane lore) DC by 2 to get the effective caster level for the spell resistance check.
Incantations as Spell-Like Abilities
Some creatures have spell-like abilities that duplicate the effects of incantations. There’s no chance of failure and no backlash for such spell-like abilities, which don’t require components of any kind and take only an attack action to activate.