Gathering Storms

Gathering Storms is a main quest

Detailed walkthrough
The quest is given upon completion of The Grand Staircase, The Advance Scouts, The Treacherous Road and River Monsters. When the Courier speaks to Daniel after these tasks have been completed, he responds with two possible options:
 * Help the Sorrows evacuate Zion Canyon, beginning the quest Flight from Zion.
 * Fight the White Legs with Joshua Graham, beginning the quest Crush the White Legs.

Selecting either option completes the quest, and also grants the achievement.