Their escape route cut off, those undead monsters were in a dilemma. Should they return to crush the enemy forces attacking their lair, or, according to the Undead Overlord's command, should they first annex Zhou Tian's territory?
Whatever the intent of these undead monsters, they couldn't possibly use their ultimate strength hidden within their large army.
Thus, considering the situation at the time, their military power became exceedingly important to these undead monsters.
To reduce the losses caused by Zhou Tian's harassment, these undead monsters even split their forces amongst various places they had occupied. They intended to depend on their formidable strength to avoid further battle damage.