Apep is the serpent god of Egyptian mythology. Though you might have heard of him by his Greek name, Apophis. (Any Stargate fans out there?) He was an embodiment of darkness and chaos, the enemy of light and order. Apep was a prominent figure in the cosmology of ancient Egypt but was a deity whom people worshiped against rather than worshiped.
It was believed that Apep lived in Duat, the Egyptian underworld and that every night he would attack the sun barque carrying the god Re and his party in an effort to swallow the sun. Some stories said that he waiting just over the western mountains and would attack the sun as it set. Others, including the Amduat or Book of the Underworld, said that Apep waited for Re in the Tenth Hour of the Night to swallow him up before dawn. Apep had a hypnotizing stare that he used on Re and his entourage while damming up the river that the sun barque traveled on with his coils. Yet Re had many defenders with him to help fight off Apep, including the god Set, and so everyday they managed to defeat Apep once again so that the sun could rise in the morning.
Though it seemed to the Egyptians that sometimes Apep could escape from Duat and attack Re during the day and even sometimes succeeded in swallowing him for a few minutes before Re's defenders cut him free. That was how they explained eclipses of the sun. Other natural phenomenon such as earthquakes and storms were also explained as times when Apep got the upperhand in his battles with Re.
In order to aid Re and the other gods in their daily battles with Apep, Egyptians priests would engage in certain rituals such as the annual Banishing of Apep. During this ritual an effigy of Apep was made and imbued with all of the evil of the land. It would then be beaten, crushed, smeared with mud and burned. There was also a text called The Book of Overthrowing Apep which gave instructions for the spells and rituals that would help defeat Apep each day.
Apep was viewed as a very powerful god and earned some awesome titles such as World Encircler, Evil Lizard, the Serpent of Rebirth (since no matter how many times he was defeated, he always came back to life), and my personal favorite Eater-Up of Souls (he was thought to devour the souls of men who traveled in Duat).
Thus Apep is the model for the creature of chaos who, along with his family, will be the primary villains in my fantasy saga. Pretty good inspiration, don't you think?