This is a development empire quest. Empire quests are activated in the campaign automatically or in a scenario by selecting the 'Empire quest' checkbox under 'advanced setup' and 'gameflow' as you set up the game.
Empire Quests can be viewed in game by bringing up the radial menu with the Y button and selecting "Objectives".
To Unlock The Harbinger:
- Build a Doomsday structure in a colony
- Launch a Doomsday operation
Rewards (For being first player to finish):
- All colonies gain 100 Happiness
- Unlock your secret tech's doomsday weapon