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There's nothing to be done here. 29,500 skeletons and 1,700 vampire lords (not counting the rest) is a significant argument. I assembled such an army through diplomacy and by taking advantage of large enemy armies. It would be better to think about how to summon a ship (I believe that if it's not on the map, summoning is not possible) or place it near the shore, otherwise, you won't be able to reach the islands.