Words are powerful things, and no word is more powerful than a true name. Every single thing which exists has a true name, but the true names of abstract things are even more difficult to master than those of living beings. The true name of a creature resonates with the very core of its being, and gives the knower ultimate power over it. A creature's true name can change, but this does not happen easily. As the true name represents their very soul, it is only by changing themselves at the most fundamental level that someone may change their true name. Examples might be a paladin renouncing his faith and changing in alignment, a villain suddenly realising the evil he has wrought and repenting, or a player character retiring from adventure forever.
The true name of a creature is as difficult to learn as it is dangerous to know. Despite this, though, many seek their own true names for their benefit or the true names of their enemies, in order to exert power over them. Though knowing a mortal's true name does not give the same measure of power over them as it does over an immortal being, it still is a powerful thing to know. Knowing your own true name, too, brings great benefit; those who know their own true name gain an insight into their very nature and destiny, and learning it instantly increases a PC's experience point total by 10% of its current value. Non-levelled NPCs who learn their true name obviously do not gain this experience award, but they are unlikely to remain unlevelled once they learn it - learning your most fundamental nature has a powerful effect on anyone, and those who learn their own true names often rise to greatness.
The process of learning a true name is difficult; many deities (though not all) have access to a large number of true names, and some in particular (usually deities of secrets or knowledge) will have the power to see the true names of most things. Most deities have access to the true names of those who serve them, and many of the denizens of the outer planes - angels, demons, devils and the like - know their own true names inherently, though it is a secret kept closely guarded. It is common for a mage to summon a demon or other extraplanar servant to find the true name of his enemy through whatever means necessary, though for obvious reasons it is unwise to allow it to learn your own true name. Spells such as Legend Lore may have some chance to reveal a true name if it is recounted in an old tale somewhere. Those capable of granting a Limited Wish can learn the true name of a willing creature, while those capable of casting a Wish may learn the true name of anything of less than demigod power, though if it is unwilling it receives a saving throw. If it passes, the true name is not learnt and the caster may never again attempt to learn that name with a Wish. Attempting and failing to use Wish to learn something's true name will always alert the target, and if they possess powers of teleportation or plane shifting they may immediately follow the spell back to the caster and learn their identity or appear before them as if through a Gate spell. Wizards or priests with Teleport, Teleport without Error, Plane Shift or Gate prepared may use one of these spells to follow the link if they make a successful Spellcraft check to cast it before the connection fades.
The benefits of knowing another creature's true name are as follows; these apply generally to all mortal beings. Any hostile spells cast upon you by a creature whose true name you know are saved against at a +4 (or 20%) bonus. Further, you may include a creature's true name when casting a spell to completely deny them a saving throw (or any other means of avoiding or reducing the spell), making them completely helpless against the magic. This increases the casting time by 1. An extraplanar creature summoned, conjured or compelled by a mage or priest may break free if they know the true name of the conjurer, leaving them free to wreak havoc upon them. These are the most easily utilised effects, but the real power of a true name lies in ritual magic; as with normal spells, including the true name of a creature in a ritual makes it completely powerless to resist it.