Augur

Augurs are eldritch mages whose power comes from other, more powerful beings. They make pacts with these beings, gaining the ability to perform a limited number of spells in exchange for their service. Occasionally, augurs try and steal magic from other beings, either through arcane rites or false prayers.
Augurs do not always know who their eldritch patron is. While most can guess at their patron's identity, or at least what their patron is, others are completely unaware of their patron's identity, and may not even be aware they've made a pact. In these cases, the augur has already fulfilled some esoteric service, and been granted their strange powers as payment without even knowing they've done so.
Broadly speaking, there are several "types" of pact that augurs may make.

Eldritch Pact (Spell List)

Agathionic Pact (Agathion): Pact made with a kirin, a morally-neutral spirit, or an azata or chalkydri
Agent of Balance (Harmonic): Pact made with a spirit of balance to maintain harmony between order and chaos
Agent of Chaos (Chaotic): Pact made with a chaos spirit to sow discord
Agent of Law (Law): Pact made with a spirit of order to impose control
Angel's Emissary (Angelic): Pact made with a holy angel of a god, typically a god of light
Archmage's Servant (Acolyte): Pact made with a powerful wizard or cabalist to better serve them
Divine Thrall (Enthralled): An unwilling pact forced upon a mortal by a hostile god, usually a god of darkness (not selectable by players)
Earthspeaker (Earth): Unorthodox pact made with earth-bound spirits, their natures are usually unclear but benign, being interested in maintaining the world
Eldritch Bride (Bride): Pact made with a powerful entity to act as a lover, concubine, wife, or sex slave; if a man is allowed to make this pact, he must accept being transformed into a woman
Eldritch Groom (Groom): Similar to an eldritch bride, except the mortal is a man and expected to perform manly duties for his patron
Fey Pact (Fey): Pact made with a good-aligned spirit, usually a totem, but can be a genie or dryad
Fiendish Pact (Fiend): Pact made with an evil spirit, such as a demon, devil, or dire wraith
Mystic Healer (Healer): Pact made with a benevolent entity, the mortal is granted healing powers to ease pain and cure illness
Spell Leech (Parasite): Not so much a pact, the mortal performs forbidden rites and false prayers that allow him to siphon eldritch power from a djinn or lower-ranking god, but at great personal risk; they rely on Stress Level instead of FP
Spirit Binder (Voodoo): Not a true pact; instead the mortal has managed to bind, or procure a previously-bound, spirit of moderate power, and can forcibly extract magic from it against its will, but must keep its container on him; they rely on Stress Level instead of FP
Unknown Origin (Mystery): The mortal has been given eldritch powers without knowing why or from what; they rely on Stress Level instead of FP

Classes

Warlocks, the default augur
Diabolists (Satanai♀ only), evil witch that gains more spells but must be her patron's slave
Mystics, augurs that record the spells they gain like an arcanist to retain some control or to learn arcane magic more quickly than true wizards
Rectors, religious augurs that gain both eldritch and divine magic from a god
Witches (Females only), gains eldritch, divine, and nature spells but is more strongly bound by her pact

Recommended Prestige Classes

Master Augurs
Chosen Ones
Dragon Disciples
Hag Matriarchs
Troll Patriarchs
Umbralites
Vampire Lords