Kali is a demon from Hindu mythology who is described as a being of great power and (in some sources) the origin of evil itself. He is the archenemy of Kalki, the tenth and final Avatar of Vishnu.
Despite sharing the same name, Kali is not the same entity as the fearsome goddess Kali, who, while vicious, is a divine being and ultimately promotes good. The demon Kali promotes only chaos and destruction, without any of goddess Kali's more positive ideals.
The Kali Yuga
Kali is the ruler of the most corrupt and violent of the Hindu cycles, in which mankind abandons the gods, and morality is replaced with open hostility and depravity. This cycle is known as the Kali Yuga, named after its infernal nature and the demon which oversees such evil.
Kali symbolizes everything that is unholy, the destruction of balance, peace and life itself. When life becomes intolerable for good people and mankind is entirely under Kali's chaotic power, it is prophecized that Kalki shall be born and take arms against Kali and his many minions, becoming the "divine chastiser" who shall punish the wicked men who thrive in the Kali Yuga at the cost of others. He shall also do battle with Kali personally and slay the monster to free the universe from Kali's cruel rule and re-establish order. This allows the coming of the Satya Yuga, a golden age where mankind is said to learn from the gods and prosper for many centuries before inevitably becoming slowly corrupt again, as is the nature of the infinite cycle that makes up Hindu cosmology.