This battle is between Necropolis and Preserve. To defeat your opponent, you must gather all the creatures you can, train your heroes, and only then can you enter the battle. However, if you try to resist, you may face opposition (rebel heroes).
Watch out for those who might attack you here and there, as they can weaken your army, which is not advantageous! If you feel everything is ready (you've recruited enough creatures and your hero/heroes are well-prepared), you can launch your attack.