Red Spotted Spiders lived in groups. They were the kind of animals that didn't act alone. They had a Spider Queen which is commonly known as the Spotted Spider Queen among cultivators.
It controlled the colony with an iron grip. No spider will disobey its orders and they would all follow it with blind obedience. Only when the Spotter Spider Queen dies, will another take its place. And only with its direction will the other spiders attack or withdraw.
The Red Spotted Spider in general had the strength of a Violet Palace realm cultivator. With the Spotted Spider reaching the pinnacle of the Violet Palace realm. Duncan had read about these rare breeds of spiders while scouring the sect annals for information about the cultivation world. That is how he knew the value of the Spotted Spider Silk as well.