They called it plundering because disciples from all the sects and even the 25 cultivation families will participate in the harvesting. At the end of the day, the secret realm would be stripped bare.
Harvesting is what the disciples of the Blood Skull sect think they are going to do. They know that they are going to a place where many people will die, but they don't think that they will die.
This is the confidence granted by strength. The only ones that disciples of the Blood Skull sect are afraid of are other disciples of the Blood Skull sect. As long as the other disciples of the Blood Skull sect don't antagonize them, they don't believe that they have anything to fear.
In general, it is considered by the disciples of the Blood Skills, etc., that the B-grade disciples of their sect are on the same level as the best disciples in other sects.
