Thanks for playing! In Necromancer's Army you army should reach the dark energy source, but castle's guards try to prevent it by attacking your zombies. When zombie reaches the source - it dies, but fills your dark energy storage. You should deliver the energy from your base to the cave to increase your blood to stay alive. You can also spend your blood on upgrades. But I tried to write all of this in the tutorial in the main men. But it seems the tutorial was unclear to you, right?