After Athena falls, the Security Team will have to seal the Bio-Domes, trapping most of the infested. In the final events of the Night of the Dead storyline, Easy Company will arrive and clear the remaning infested from the Bio-Domes.
This seemingly simple task is actually extraordinarily difficult given the time constraint and massive hordes of infested. The team may choose to move together, or send two players to attempt to seal the domes. This is also the last chance to find items or complete side missions.
The infested hordes are made up of some of the strongest enemies including Agrons, Devourers, and Infested Marines. The Devastators are helpful if the players attempt to move as a team. Each dome will have two hexes. Two players will need to stand on the hexes simultaneously to seal the dome. This absolutely requires two players minimum. Once the players stand on the hexes the Bio-Dome will seal instantly. There is not a sealing time duration, so Inception can be quite helpful to run in and seal it quickly.
Time is extremely limited. If the team takes too long they will initially receive -1 rating for mission delay. This will soon be followed by -2 rating for mission delay. After this, the hordes will subside, and sealing the Bio-Domes becomes much easier.
Because this mission is extremely difficult, teams will sometimes choose to hole up and take the -3 rating. Then, two players can move out and easily seal the Bio-Domes after the mission delay. Before sealing the last dome, be absolutely certain everyone is ready to proceed.