The Elder Scrolls Online: Firesong, the last DLC in the Legacy of the Bretons storyline will release on November 1st for PC and on November 15th for console players. Ahead of that date, Bethesda has launched the prologue today which allows players to get a sneak peek into the storyline for the DLC.
The prologue has already started and all you need to do is to pick up the Sojourn of the Druid King Quest in the Crown Store.