Sanctuary – “Crimson Tide” [Campaign 4 – Mission 3]

Hello. People keep asking the same questions, but I haven't seen a clear answer yet!!! The same problem exists with this mission. On the map, besides me and those ships, there is no one left; I have defeated everyone. 3 out of 4 towns have been rebuilt; the 4th (Necropolis) cannot be rebuilt, the button is not active. The quest says that 2 out of 4 villages are under control, but it's unclear why, since I am the only one left on land. If not me, then who are they subordinate to? It's unclear. If anyone knows how to complete this mission, PLEASE help!!! The text says that the Necropolis fleet will be destroyed by the tide, but how do you trigger this tide if the buildings cannot be rebuilt??? I don't know what to do anymore. HELP, PEOPLE!!!