Proxy spells are one of the three forms of supernatural aid given to the Lothanasi and other disciples of the Pantheon. For the other two types, see the articles on inherent magic and divine intervention.
A proxy spell is the magic of a deity channeled through a mortal caster. Clerics pray, meditate, or otherwise focus their minds at the beginning of each day, and divine energy flows into them from their gods and is stored for later use. If a cleric comes upon a situation where a particular spell is needed — and it is one of the spells that the god has granted him access to — then the cleric can access the spell through the use of an invocation, a type of ritualized prayer (usually spoken in Old Tongue or Elven). The god's pre-stored power flows out of the well within the cleric, without any conscious effort on the god's part, and is channeled into the spell by the cleric, who shapes the particulars of where it is to be cast, and upon what recipients or targets it takes effect. The god is aware of which spell has been used and by whom, and may choose to investigate the circumstances in which the power has been used, but the god's active participation or attention is not needed in order to perform the spell. The cleric is in control of the spell, but casts it as a proxy for the presence of the god himself.
Proxy spells were an exceedingly useful invention from the pantheon's point of view; they allow the gods and daedra lords to project their will into many places at once. Since the members of the pantheon are neither omnipresent nor omniscient, proxy spells allow them to take their minds off of the more mundane applications of their power and focus on the larger plans that they have in play. As convenient as they are for the gods, however, they are even more useful for the clerics; proxy spells are the tangible power of the gods at work in the world, but at work in such a way that the clerics are able to be agents of change rather than mere supplicants. The invention of proxy spells allowed clerics to begin to use the gods' power in the ways that they saw fit, without constant supervision or the need to ask for permission, which allowed them to be more proactive and creative in serving the needs of their communities (or the advancement of their own agendas).
While the cleric need not justify every individual use of proxy magic, however, she is being watched and evaluated in how she uses the magic given to her. Some of the pantheon's members are fickle beings, and they may choose at any time to extend or withdraw the use of spells if their opinion of the cleric's behavior changes. A cleric who ventures too far from a particular god's ethos or fails to pay him proper honor will find that the god has withdrawn his support from her; conversely, a cleric who proves herself faithful and dependable is entrusted with greater power and more formidable spells.
A god can only grant the power to cast spells that fall within his/her portfolio; thus, disciples of a particular deity are limited to only those spells that are within the purview of that deity.1 The Lothanasi can potentially have access to the spells of all nine Gods of Heaven, and the Moranasi can demand limited access to spells from any of the nine Daedra Lords, but a God or Daedra Lord who bears a grudge against that particular cleric can still choose to deny her access to his spells, particularly if the deity's complaint is a reasonable one that can be justified to Kammoloth or Ba'al, respectively.
The ability to cast proxy spells appears to be completely unrelated to skill with arcane magic. Every individual mortal seems to have a certain level of capacity for storing divine energy within himself, as well as a degree of receptivity to accept and channel such magic in the first place. Those whose receptivity is higher and whose capacity is greater have greater potential to become clerics; since capacity and receptivity tend to grow with the life of rigor and discipline that the priesthood requires, older and more experienced clerics become able to channel more and more powerful sorts of spells. Regardless of a cleric's age and experience, though, the amount of power available for proxy spells is always limited; once the cleric's reserve has been exhausted, he must rely on other skills — or hope for a miracle of divine intervention.2
Generally, a cleric is aware of which proxy spells have been given to her (or taken away from her) long before it becomes necessary to use them; any changes in a cleric's access become clear when she meditates, prays, or focuses her mind to receive her allotment of divine energy. However, there are certain rare instances in which a creative and talented cleric is able to shape the god's invested power in a new way to suit a particular need, spontaneously and without any prior preparation. Merai did this when she invoked Yajiit's power to create a spontaneous sunlight spell to destroy a powerful vampire.3 Such incidents are always a cause for great interest among the priesthood, since they generally indicate that the cleric in question has a special bond with that particular deity.