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

The following helped me when all castles were captured in the quest: only three were displayed. I skipped a few turns, and undead emerged from the vortex. I allowed it to capture a castle, and when it moved further, I recaptured the castle, and so on, one by one. In the end, I activated all four altars. Give it a try!