In addition to the Moiræ, who presided over the life of mortals, there was another divinity, called Ker, appointed for each human being at the moment of his birth.
The Ker belonging to an individual was believed to develop with his growth, either for good or evil; and when the ultimate fate of a mortal was about to be decided, his Ker was weighed in the balance, and, according to the preponderance of its worth or worthlessness, life or death was awarded to the human being in question.
It becomes evident, therefore, that according to the belief of the early Greeks, each individual had it in his power, to a certain extent, to shorten or prolong his own existence.
The Keres, who are frequently mentioned by Homer, were the goddesses who delighted in the slaughter of the battle-field.
From: Berens, E.M. The Myths and Legends of Ancient Greece and Rome. New York: Maynard, Merril, & Co., 1880. Text in the public domain.