Notes on Magic
While magic in general is respected in Moonthrone (the city was after all founded, and supposedly still ruled, by a wizard), it also largely feared and mistrusted by the common rabble of the City Below, primarily due to the unique danger warlock pact magic poses to the the City.
Something is peculiar about Moonthrone and the lands surrounding it. The boundaries between planes are thin there, allowing otherworldly entities greater access and influence on the material plane, and drawing many of them to pact with mortals in efforts to exploit that weakness. Warlocks may in some cases find their magic more potent in Moonthrone, but not without extra risks (see extra rules for Warlock Pact Magic).
Warlock patrons may leave their mark on reality by corrupting the warlock or even the world itself when a warlock invokes his patron and gives it a foothold on the material plane. If a patron gains too great a foothold, an Incursion may occur.
Incursions are areas of the material plane that have been corrupted (possibly permanently) by exposure to extraplanar energies. The effects of the corruption vary depending on the type of being and realm (corruption caused by a greater fey may lead to strange fungal growths, or an area where time passes at a faster rate, while demonic corruption may cause buildings to sweat blood, or a city block to be tormented by anguished screams of unidentifiable origin, for example), but are potentially permanent, and cause those who stay in the effected area to become corrupted from exposure as well. The worst incursions, such as the unfortunate Wyrd District or the Stormstreet Incursion, are so corrupted by conflicting otherworldly powers of various orgins that they are essentially areas of pure random chaos. So far, Moonthrone's rulers have found no way to cleanse or reverse Incursions. The most effective response seems to be quarantine. When an incursion is identified, the City Watch works quickly to wall up and prevent all travel into or out of the effected area, upon penalty of death.
Due to the dangers of pact magic, the rulers of Moonthrone have enacted rules to regulate magic in the City. Access to Moonthrone is regulated largely due to the dangers of warlock corruption, with all who pass through the city's gates inspected for signs of witchcraft and warlockery (as pact magic is often referred), and only registered and documented magic users allowed to practice their craft.
There are two major exceptions to the rules banning pact magic: devils and Acolytes of Mourn.
Due to the Hellish Treaty over 133 years ago, approved devils may pact with mortals, with the understanding that they are not allowed to interact with the material world directly via invocation.
Acolytes of Xarog Mourn
The Wizard King Xarog Mourn founded Moonthrone over 1000 years ago, and personally raised the City Above with his awe-inspiring power, or so the legends say. He also supposedly still lives, and studies in his central palace tower in the City Above. Apart from ill-omened visits from the Emissaries of Xarog Moun (mysterious Aaracroa of grey to black plumange who sometimes deliver messages directly to the Spire Lords from the Wizard-King, and are the only creatures ever known to return from visits to Xarog Mourn's palace), the best evidence that Mourn is still alive are the Acolytes of Xarog Mourn, warlocks exclusively from the nobility in the City Above who supposedly gain their spellcraft directly from Moonthrone's founder. They are also the most commonly seen aristocracy in the City Below, as the white-robed acolytes are largely responsible for magical interrogations for licensing purposes at the city's entrances.
More on Hellish Pacts
The Church of Asmodeus, based out of Devil Town, offers an exception to the unlawfulness of conspiring with otherworldly entities in Moonthrone. With a valid license, a warlock may pact with an approved Lord of one of the Nine Hells, as long as the contract is authorized by the Church. Contracts between the individual warlock and the patron vary in each case, depending on the whims and sensibilities of the patron devil (and the negotiating skill of the warlock). It is not uncommon for a contract to have at least one esoteric and obscure requirement, such as a duty to nail a toad to his front door every third sunrise, or to abstain from kissing a redhead, with repercussions as varied as forfeiture of the contract, to bleeding gums, or death. Whether fiends gain any actual benefit from these clauses or simply get off on being controlling is a matter of debate among theologians and scholars. In each contract however, the patron herself is separately bound by an agreement with the Church and the City not to breach the planes in response to invocations.
Warlock contracts with authorized devils differ from oaths taken by clerics and priests who serve fiendish lords, in that they are individual agreements between a warlock and a devil with clear, negotiated terms, as opposed to oaths or tenets to which a priest adheres in order to gain unholy clerical abilities. As he levels up, the warlock gains more powers and spellcasting abilities as he renegotiates the contract for greater uses of the patron’s power. It is also not uncommon for warlocks serving the archdevils to change patrons as they gain (or lose) power and prestige; the politics of the Nine Hells are fickle, and a warlock may find himself dropped for failure to further his patron’s fortunes, or poached by a more powerful devil who wants to take advantage of a warlock’s notoriety or to stick it to a rival.
While sorcerery is not full-on outlawed in Moonthrone, sorcerers are looked at with more scrutiny than your typical wizard who attains spellcasting through rigorous study. Since the gift of sorcery often results from exposure to otherworldly energies, sorcerers are rigorously tested for signs of corruption before being licensed and allowed to enter the city, typically with a 3 day quarantine period at one of the forts at the city gates (Fort Rustgate to the north and Eastgate in the Pelt District), or on board their ship if entering by river or sea.
Furthemore, sorcery licenses have limited expiration dates, and must be re-upped every 3 months, with further inspection to check for corruption.
Also of note, sorcerers may call upon their inner magic to boost their spells, much like a warlock boosts his spell with his patron's help (see Warlock Pact Magic). A sorceror may do so using sorcery points with no ill effect, or push further with any ability that uses sorcery points once he is out of sorcery points, but doing so comes with a cost--the sorceror must roll for corruption, and risk an incursion at DM discretion (you never know who or what may be watching just beyond the boundaries between worlds).