The Necromancer's Arrow Tower was actually a Magic Tower, and on this Magic Tower, the Necromancer's mana never ran out, allowing him to continuously cast the Death Cloud ability.
On the one hand, Death Cloud was a natural talent ability of the Necromancer, on the other hand, besides providing endless mana, the Necromancer's Arrow Tower also enhanced the effect and damage of the Death Cloud, making a single Wizard Tower more useful in war than two automatic crossbow Arrow Towers.
Song Jian flapped his Flame Light Wings, deftly dodging the attacks. Although the Death Cloud had a wide attack range, it only exploded in a large area after reaching its target; in reality, what the Necromancer shot were merely energy balls the size of basketballs.