Carcom was surprised because even though he had lived in the dragon clan for centuries, he was unaware of the existence of powerful warriors like the 5 dragon kings.
This lack of information almost led to his death. It was all part of the dragon ancestor's plan. These dragon kings had remained hidden with Arion and his daughter Lilith for eventualities like this.
He wanted to lure Carcom out and eliminate him with his exit, but he never expected Carcom to be strong enough to escape them. Defeating them was certainly impossible, but escaping was possible.
Despite managing to escape, Carcom suffered injuries and they were not light, but as long as he was alive it was a gain for him. Unfortunately, his plan had major problems.
Not only did he return empty-handed and almost die, but his absence made the leaders of Purgatory believe he was a traitor. Certainly, Carcom was not loyal to the organization, but having them as enemies was not appropriate, at least not for the moment.