The elves were the first to utlitize magic. Their folklore is filled with stories about ancient heroes who somehow managed to steal magic from the gods (whether by trickery or force). Regardless of how they attained it, the elves began to use the elemental powers of the planes for their own gain. Before they gained too much power, the god Keyldynan intervened and taught the elves how to responsibly use their new powers.
Whenever a spellcaster uses arcane magic, they siphon off power from the other planes of existence to bring those energies into the Material Plane. These incantations are but a fraction of the power that the gods themselves wield, but it is still a godlike act of creation. The Material Plane itself was brought into being by the gods using similar powers. The essence of several planes was melded together to create the world that all mortals inhabit. The gods took umbrage at the mortals using their powers recklessly, so they unleashed the fog upon the world.
Divine power is derived from the gods themselves. Whenever a mortal proves particularly faithful, the gods grant them a sliver of their own power. Thusly, divine casters act as conduits for the power of their deity. Some faithful servants are even exalted to avatars of their deity, and they are conferred truly legendary powers.
Though the power source of arcane and divine magic is the same (the other planes of existence), arcane spellcasters will their magical effects into being on their own. This is seem by the gods as the theft of their power. This theft is why some believe the Humbling was enacted, but some say that the gods were not simply punishing hubris. Some believe that the intense use of arcane magic was depleting the power of the other planes and creating instabilities on the Material Plane. Some believe the Humbling was a desperate action to curb the overall number of arcane casters.
Despite the causes of the Humbling, the vast majority of casters died, and their secrets died with them. Spellcasters are exceedingly rare in Faerstice. Most arcane practitioners choose to hide their powers for fear of persecution. This paranoia is validated by the fact that many are fearful of spellcasters and strive to hunt them down due to fear of divine retribution. Most common people see the unrestrained exercise of arcane power as the downfall of mortal beings, and they will do anything to stop another event like the Humbling from taking place. Divine spellcasters are less feared, but most distrust the gods and, by extension, their servants.
There are some exceptions. Each major city has one group or another that bands together to dabble in the arcane arts. However, any such groups are highly regulated and watched over with the believe that restrained spellcasting will not bring the ire of the gods. The scholars of the Wizened Tower in Rulathar, for instance, work with arcane magics to protect the mortals of Faerstice from the fog and lesser threats.
Occasionally, some mortals are born with a touch of the divine, and they can simply will the planar forces into being. These children are killed in some areas to appease the gods, but some areas foster their tutelage by sending them to live in monasteries or academies.