Short version: They can be given as a result of deities wanting one of their priests to go on a vital quest.
Long version (spoilered due to space)
Quest Spells (from the Tome of Magic)
Priests and clerics are the servants of Powers - immortal entities with abilities far beyond those of mere mortals. Yet these servants do not wield magical forces equal to those of wizards; priests have nothing to compare with the wish spell, for example. Circumstances will arise when a priest should be able to call upon the magical energies controlled by his Power to achieve something extraordinary in serving a sacred duty. Quest spells are designed to satisfy these extremes and allow the priest to wield high-powered magic without drastically altering the scope of his magic. Quest spells are a category of powerful spells without an assigned level. They should not be confused with the 5th-level spell quest, which is a specific single spell.
While quest spells are powerful, they are not as powerful as the energies used by Powers. If a god chose to flatten a mountain or raise an island, he could probably do so. Priests cannot achieve such huge effects; they are still mortal beings. But quest spells do provide a priest with magic more powerful than any other priestly magic; a quest spell could easily mean the difference between success or failure in a mission. Quest spells are capable of affecting large areas or numbers of creatures and allow the shaping of great energies; they are often difficult or impossible to resist or dispel.
Quest spells are not part of a priest's normal repertoire. These spells are granted powers, bestowed directly by one's deity to achieve special goals.
Why Quest Spells? Two circumstances are most likely to warrant the granting of a quest spell to a priest. First, a Power may contact the priest in a dream or omen, or by sending a servant or avatar. In this case, the Power requests that the priest perform a vital service on behalf of the Power (the nature of such a request is discussed later). The priest is effectively commanded to go on a quest-hence, the generic title of quest spell.
A second case for the granting of a quest spell may occur if a priest were to discover something of fundamental importance to the faith which the Power must be appraised of (not all powers are omniscient). A priest contacting the Power (with a commune spell or by prayer) might beseech the Power to grant him some exceptional magic to address the situation. The request for a quest spell must never be motivated by selfish considerations on the priest's part (such hubris is grossly offensive to any Power), and circumstances must be truly exceptional. The Power then considers the priest's request and responds accordingly.
Comments
Short version: They can be given as a result of deities wanting one of their priests to go on a vital quest.
Long version (spoilered due to space)
Quest Spells (from the Tome of Magic)
Priests and clerics are the servants of Powers - immortal
entities with abilities far beyond those
of mere mortals. Yet these servants do not wield
magical forces equal to those of wizards; priests
have nothing to compare with the wish spell, for
example. Circumstances will arise when a priest
should be able to call upon the magical energies
controlled by his Power to achieve something extraordinary
in serving a sacred duty. Quest spells
are designed to satisfy these extremes and allow
the priest to wield high-powered magic without
drastically altering the scope of his magic.
Quest spells are a category of powerful spells
without an assigned level. They should not be
confused with the 5th-level spell quest, which is a
specific single spell.
While quest spells are powerful, they are not as
powerful as the energies used by Powers. If a god
chose to flatten a mountain or raise an island, he
could probably do so. Priests cannot achieve
such huge effects; they are still mortal beings. But
quest spells do provide a priest with magic more
powerful than any other priestly magic; a quest
spell could easily mean the difference between
success or failure in a mission. Quest spells are
capable of affecting large areas or numbers of
creatures and allow the shaping of great energies;
they are often difficult or impossible to resist or
dispel.
Quest spells are not part of a priest's normal
repertoire. These spells are granted powers, bestowed
directly by one's deity to achieve special
goals.
Why Quest Spells?
Two circumstances are most likely to warrant
the granting of a quest spell to a priest. First, a
Power may contact the priest in a dream or
omen, or by sending a servant or avatar. In this
case, the Power requests that the priest perform a
vital service on behalf of the Power (the nature of
such a request is discussed later). The priest is effectively
commanded to go on a quest-hence,
the generic title of quest spell.
A second case for the granting of a quest spell
may occur if a priest were to discover something
of fundamental importance to the faith which the
Power must be appraised of (not all powers are
omniscient). A priest contacting the Power (with
a commune spell or by prayer) might beseech the
Power to grant him some exceptional magic to
address the situation. The request for a quest
spell must never be motivated by selfish considerations
on the priest's part (such hubris is grossly
offensive to any Power), and circumstances must
be truly exceptional. The Power then considers the
priest's request and responds accordingly.
Edit: Ninja'd by someone who actually had an idea about this… Nice.