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At the beginning of each day, following a normal period of rest, you can dedicate spell slots for the day to holding metamagic effects for later use. The spell slot dedicated must be of a level at least equal to the level penalty associated with the metamagic effect to be stored. So, for example, a sorcerer with the Maximise Spell feat could choose to store it in a spell slot of 3rd level or higher. A spell slot dedicated to storing a metamagic feat in this fashion cannot be used for any other purpose, it is considered to be a precast spell. In addition, once it cradled, the metamagic effect cannot be altered, so a cradled Maximise Spell metamagic feat cannot be exchanged for an Empowered Spell feat.
The benefits of cradling metamagic effects are two fold. First, when you wish to modify a spell with a cradled metamagic effect, the casting time of the spell is not modified. So, a fireball spell modified by a cradled Maximise Spell feat requires only a single action to cast, rather than a full round. Second, a spell modified by a cradled metamagic effect does not require a higher level spell slot. So, for example, a magic missile spell modified by a cradled Maximise Spell feat uses a 1st level spell slot and the 3rd level spell slot, rather than a 4th level spell slot. Cradled feats and the slots that store them, are expended after a sin