The few undead who arrived at this ancient ruins space did not attack Zhou Tian right after their entrance. Even when they had the ability to rescue their minions, they watched as Zhou Tian slaughtered them. All they did after entering the ancient ruins space was to fortify a base, seemingly preparing to seize a foothold for their own forces.
The reason why the previous force set such a trap for Zhou Tian was not only because their trap was effective and could easily deceive Zhou Tian, but mainly because Zhou Tian directly blocked their 'door' to the ancient ruins space. Under such circumstances where they couldn't deliver large quantities of troops through the spatial passage, they were killed one by one. Even if that force knew that only Zhou Tian blocked their passage, they still found it difficult to demonstrate their numerical superiority.
However, that all changed radically after they secured a foothold.
