This is a military 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 Invader:
- Declare war on an NPC faction
- Kill 2 armies belonging to an NPC faction
Rewards (For being first player to finish):
- Receive a tier I unit at the capital colony