With the Corpse Soul Lord's help, Zhao Fu quickly found where the two other Corpse Soul Lords were. Zhao Fu did not bring any soldiers with him because he did not want a direct fight. Instead, he wanted to sneak in and assassinate the Corpse Soul Lord.
After coming to a Corpse Soul City, Zhao Fu found that there were many Corpse Soldiers defending it warily. The Corpse Soul Lord had most likely ordered them to defend as it recovered.
Back then, the two Corpse Soul Lords' wounds were even worse than the female Corpse Soul Lord's.
Zhao Fu held the Corpse Soul Sword and snuck into the city. If he had his Assassin profession, things would have been much easier. However, even if his Assassin profession was sealed, he still had his experience of being an Assassin.
Adding on the fact that these Corpse Soldiers were quite stupid, it was quite easy for Zhao Fu to sneak into the city.