The flying knife looks like it's a zhenbao, a treasure forged using specific bloodline techniques. This is a different kind of treasure from the other kinds of weapons that are controlled remotely. It's also stronger than other weapons because it can use the power of the person's bloodline. The kid is so excited that he grabs the knife and tries to stab it, but the knife won't listen to his command. He grabs the black stone and tells the kid to spread it out and kill him. He's going to kill him anyway.