Eldest of the Gods
The only surviving historical account of any beings described as “Titans” is Hesiod’s Theogony. It was written sometime in the 7th or 8th century B.C. and described the geneologies of the Greek gods and their war with the Titans called the Titanomachy.
In Hesiod’s work, the Titans were defeated and imprisoned in Tartarus. While these are the same beings which have escaped and are currently waging war on the gods, they are not specifically bound to Greek mythology.
The Titans are the personified primal forces of the universe. Often too immense and powerful to act personally, they act through avatars or their spawn. After escaping Tartarus, these beings seek revenge on all of the gods for their imprisonment.