Gathering Storms

Gathering Storms is a main quest in the Fallout: New Vegas add-on Honest Hearts.

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.

Telling Daniel which option the Courier chooses completes the quest.

The achievement is obtained whichever option has been chosen.