Today's question (demand, really) is short and to the point: Tell us about Apep! We are all too happy to talk about Apep (probably closer to Apapi in ancient Egyptian), also known in Greek circles as Apophis, the dread serpent of darkness and primordial chaos who lurks in the unfathomable depths of the shadow world of Duat. It is a big deal in Egyptian mythology, so let's learn about it! (Carefully, and quietly. We don't want its attention.)
Apep - often called "the Apep" in Egyptian texts, to distinguish it from lesser serpents that might be considered its offspring or reprsentatives - is an enormous snake that lurks in the darkness of Duat, sometimes said to lie coiled around the fabled Mount Bakhu, at others believed to lie in the fathomless dark waters of the primordial Nun, the only place infinite enough to contain its endless bulk. According to the Pyramid Texts, Ra, the sun god, travels through Duat each night, dying when the sun sets but resurrecting himself at the dawn to allow the sun to be reborn the next day, but Apep lies in wait there to try to stop him, appearing in the dark underworld skies with hordes of evil minions and lesser serpents.
What exactly Apep's aim here is differs slightly with the version. In some myths, it wishes to devour the sun, swallowing Ra and the solar disc into its endless gullet to become its sustenance alone; in others, it seeks only to prevent Ra from ever raising the sun for the next morning, to which end it employs any means, up to and including destroying him and his protectors before they can escape Duat to take to the skies again. In all cases, Apep is a symbol of the chaos that lurks at the edge of the universe; if it succeeds in preventing the sun from rising, it will destroy the natural order of things, and in return the world will be plunged into chaos and death.
Apep's origin was for most of Egyptian religion not recorded in any way; it had simply always existed, much like the other primordial elements of chaos and creation, and not even the most ancient of gods could claim it as offspring. In later myths, after Greek and Roman influence had caused etymological confusion that related the serpent to the word for spitting, it was said to be created by Neith, the ancient goddess of the waters, who had spat it out as unclean when she was creating the rest of the great waters. Other serpents that may or may not be related to it are found throughout Duat, including fire-breathing torch-serpents that light the solar barque's way, great serpents whose guts the sun god must travel through, and other dangerous snakes who may be offspring of Apep or simply other forms of its malevolence seen throughout the underworld.
Because Apep is so terrible (always described as fathomlessly huge, which is why it is depicted with such tightly-packed coils to represent how much of it there is, and smetimes said to have a head made of vicious hard flint or eyes of gold) and so dangerous, the sun god was understood to not be able to fight such a fearsome foe alone. The solar barque was staffed with a defense team intended to help him fight Apep off each day, including Horus, god of kingship and warfare; Maahes, lion god of slaughter and courage; Serket, scorpion goddess of poisons and disease; Mehen, the serpent god who opposes his fellow snake; and the trio of magical gods Hu, Saa and Siu, who support and enhance the magical powers of Ra himself. Each day, through their combined efforts, Apep is defeated or turned aside and Ra is saved to begin the day anew, although not without great hardship and injury to all.
The foremost of Ra's defenders, and the only deity who stands as a true opponent who can face Apep's might, is Set, god of the deserts. Apep's powers include a terrible hypnotic gaze that can freeze even a god so that they lose all reason, not to mention its unending coils which are wrapped around the barque itself and all the gods within, and thus many of the defending gods are paralyzed by its gaze each day, or physically restrained so that they cannot help Ra as they intend to. Set, however, is the strongest and most powerful of all the divine warriors, and he is unaffected by any of Apep's dangerous skills; he is too strong to be held in its coils, and too loyal and determined to be hypnotized, which allows him to take over and drive the barque for Ra while the sun god fights the snake's influence himself. He then fights Apep with a great iron spear, eventually skewering it and driving it off until the great struggle begins again the next day.
Apep's not just a big mindless sun-eating snake, either; when it comes to Set, it's downright snarky, trading insults and one-liners back and forth with the god of chaos, including making fun of his missing testicle. So clearly there is plenty of malevolent intelligence behind those great serpentine eyes.
In spite of the efforts of Ra and his fellow gods, Apep's attacks are not always in vain. Occasionally, it successfully swallows the sun; this is the case when eclipses occur, which were believed to be the result of Apep performing a surprise attack during the daytime, and which last as long as it takes for the gods to come to Ra's rescue and fight the snake off before the day can resume, and on at least one memorable occasion, the sun didn't rise at all thanks to Set being called away to the birth of his son, leaving the barque woefully underdefended. Apep was ruinously wounded when Set returned and carved a hole in its throat to let the barque through, but we must assume it enjoyed the victory while it lasted.
Apep's relationship with Set is a complicated one, and not just because of the constant insult war and physical altercations; in later times, Greek and Roman writers actually conflated the two gods into a single person, a move that would have been vastly confusing to those Egyptians worshiping their earlier incarnations centuries before. Because Set and Apep were both used to represent chaos or at least opposition to order in Egyptian myths, and because Set was so heavily demonized thanks to his rebellion against his brother Osiris (exacerbated by Greek writers conflating him with their own most evil antagonists of the gods, such as Typhon), over time the god who was the most staunch opponent of the serpent became considered just as evil, and eventually as the serpent itself. It probably didn't hurt that the Hyksos, a Semitic people from the northeastern Middle East area, had conquered Egypt at the end of the 14th dynasty (with a lot of traumatic new inventions like chariots that were not seen as very sporting) and took on Set as their patron god, which associated him with divisive invading conquerors.
So Apep absorbed Set, near the end of the Egyptian religion, and then was in turn absorbed into the Greek figure of Typhon, in a big scary stew of darkness and monsterness that threatens the universe.
Regardless of who or how it's associated, the Apep is the most terrible of all Egyptian enemies of the gods, and a figure to be feared for all of time, as long as the universe still remains in existence and the gods wish to keep it from being destroyed.