Torture
"So, what are you going to do?" Nuna asked Klaus after knocking her mother out cold. Two days ago, Klaus had sat down with them and asked for every detail about their mother, who was also their slavemaster.
Naturally, they were more than happy to sing like birds, giving Klaus all the information he wanted. With that, he devised a strategy involving the creation of a potent poison meant to strip someone of their cultivation base for a few minutes.
This poison had been taught to him by Yuying during their time—or rather, during his time as Fruity with her—in the Tormented World.
He had asked Madam Fei for help in gathering the ingredients a few months back. As promised, he didn't require the twins to do anything other than escort him to their home, where he would lie in wait for his target to return.