Divine realm magic
Regent priests primarily use their temples to tend to the spiritual well-being of those that follow their deity and to exert political influence to further the dogma of their church. However, the divine energies channeled by a single cleric, no matter how devout, pale beside the potential energies focused by the massed faith of hundreds of faithful worshippers brought to the peak of religious fervor through weeks of ritual, fasting, and prayer.
Casting divine realm spells
When a divine spellcaster casts a realm spell, she marshals her temple's followers in a massive ritual designed to channel massive amounts of divine power. The priest spends the marshalling time traveling throughout a province to speak to the devout and lead rituals, blessings and prayers at each of the various cathedrals, temples, and shrines within the province.
When a regent priest casts a realm spell, he marshals the belief of the faithful over the period of a month. The rituals associated with clerical realm spells tend to be elaborate affairs, requiring costly vestments, rare incenses, and valuable sacrifices. If nothing else, the value of such sacrifices imparts to those witnessing the importance of the cause for which such faith is mustered. During this marshalling, the regent must remain within the province and spend a great deal of the time and effort in the organization and implementation of the many rituals required to invoke divine realm magic. During the month, the priest spends the majority of each day leading worship and ritual services. Near the end of the month-long effort, the priest heads a lavish ritual in which a large number of devout worshippers provide the faith which, through the power of the regent priest's divine bloodline, is channeled into a divine realm spell.
Only blooded divine spell casters are capable of channeling divine realm magic; an unblooded priest lacks the necessary link to the divine required to focus the belief of the masses into the casting of a realm spell. Blooded rangers, paladins, and other minor divine spell casters are capable of casting divine realm spells, but due to the spell level requirements, many realm spells are beyond them. Some divine realm spells may only be available to priests of specific religions.
In order to successfully cast a realm spell of a given level, the regent priest must be able to cast conventional spells of that level. Furthermore, the regent priest must claim a temple holding equal to or greater than that required to cast the spell.
Variant: Using the power of the earth
Druids may cast spells using temple holdings. In addition, however, druids may draw upon the natural power of the earth to empower their spells. Druids are able to cast divine realm spells as if the unclaimed source holdings in the province were temple holdings under their control. The druid must have a temple holding in the province in order to use this ability. Dwarven priests of Moradin may use unclaimed source levels of mountain provinces in exactly the same way as druids.