Semper Invicta

Semper Invicta is a Brotherhood of Steel main quest and achievement in Fallout 4.

Detailed walkthrough
Once the Survivor completes the quest Call to Arms, this one will show up automatically, asking the player character to go back to Cambridge police station and talk with Danse.

After the discussion, Danse asks the Sole Survivor to help both Rhys and Haylen doing their respective job.
 * Cleansing the Commonwealth (repeatable)
 * Quartermastery (repeatable)

The quest will complete once the requisite quests have been completed at least once each. The requisite quests are repeatable whether or not this quest has been completed.

Once the job is done, the quest The Lost Patrol will begin.

Behind the scenes
Semper invicta is Latin for "always invincible."