Save Treviso or Minrathous

Reach the Point of No Return: Complete main quests in Treviso and Minrathous until you undertake "In Peace, Vigilance" to get Grey Warden help.

Recruit a Grey Warden: Complete the quest "A Warden's Best Friend."

The Choice: After "A Warden's Best Friend," Lucanis and Neve will inform you of simultaneous dragon attacks on Treviso and Minrathous. You must choose which city to defend.

Quest Branching:

If you choose Treviso: Lucanis joins you, and the quest "On Deadly Wings" begins.

If you choose Minrathous: Neve joins you, and the quest "On Blighted Wings" begins.

Consequences:

Companion Departure: The companion not chosen to accompany you will leave your party temporarily to deal with the fallout in their city. They will be colder towards you initially.

Romance Options: Neve can still be romanced if you save Treviso, but it will be harder. You cannot romance Lucanis if you do not save Treviso.

Faction Impact: The neglected city's faction (Antivan Crows for Treviso, Shadow Dragons for Minrathous) will be severely diminished. Their faction merchant will be gone for the rest of the game, later replaced by a wisp in the Crossroads with no ability to sell valuables to raise Faction Strength. This makes maxing the neglected faction's strength difficult.

City State: The neglected city will be blighted (Treviso) or dominated by the Venatori (Minrathous), which are largely cosmetic changes.

Follow-up Quests: You will receive quests in the neglected city to deal with the aftermath, such as "A City Blighted" (Treviso) or "A Shroud Over the Light" (Minrathous), to track down missing faction members.