Apep, also known as Apophis in Greek, was a serpentine god of evil, darkness and chaos in Egyptian mythology. He hates and is the enemy of light and Maat. Apep was viewed as the greatest enemy of Ra, and always opposes him and constantly battles him, losing each time. Because Apep opposed Ra, he earned the title "Enemy of Ra".
As the personification of all that was evil, Apep was seen as a giant snake or serpent, leading to such titles as Serpent from the Nile and Evil Lizard, but over the years came to be viewed as a dragon. Some elaborations said that he stretched 16 yards in length and had a head made of flint. He was also known the "Lord of Chaos".
Since everyone can see that the sun is not attacked by a giant snake during the day, every day, storytellers said that Apep must lie just below the horizon. This appropriately made him a part of the Egyptian underworld, the Duat. It was thought that his terrifying roar would cause the underworld to rumble.
Ra was assisted by a number of defenders who traveled with him, including Set, Isis and possibly the Eye of Ra. Apep's movements were thought to cause earthquakes, and his battles with Set may have been meant to explain the origin of thunderstorms. In some accounts, Ra himself defeats Apep in the form of a cat.
Tales of Apep's battles against Ra were elaborated during the New Kingdom. He was worshiped against in ancient Egypt.
Digital Devil Story 2: Warrior of the Demon City
The Apep are small snake-like servants of Set that can burrow into the human brain and take control of that person. When visiting Akemi Nakajima's mother, Ohara leaves behind an Apep, which takes control of Nakajima's mother and attacks him when he returns home. Nakajima's mother, and the Apep inside her, was incinerated by Yumiko Shirasagi when she arrived at the scene.