This mission is enabled after Moon Data Analysis research, to show that there's a principal gate site right beside the Beastmen fortress, on Elysium mountain.
Objective: Destroy 4 gate mechanisms and left.
Enemies: High number of enemies spawn from the fortress building. Other than Boss, you can meet all the Beastmen war beasts and troopers. Potentially endless.
Terrain: a combination of gate maps with a side of a huge fortress building.
Note: after this mission, if Harald is alive, he will announce his retirement and Olga coming in with Fast Learner training allow she harvest more xp and train easier. Still Harald is usually a constant present in active squad, his kill count is massive and unreplaceable. So over all a net loss in term of combat abilities. The good side is that you now can have an available spot for soldier/scientist or soldier/technician who is aching to join.