The Age of Ancients is an era in the lore of Dark Souls.
In this era, the world was ruled by the Everlasting Dragons. Humans and other creatures have yet to make an impression upon the world. It was during this time that the Lord Souls were found, near the First Flame. The three Lords that found them (Nito, Gwyn and the Witch of Izalith) formed a coalition together and used their newly discovered souls to challenge the Dragons, starting a very long and destructive war. However, there was a fourth Lord Soul, the Dark Soul, which was found by the Furtive Pygmy. The only known act via this Lord was the fragmentation of their soul becoming the birth of Humanity.
The victory of the Lords in the war marked the end of the Age of Ancients and the beginning of the Age of Fire.