Zidian is a powerful magical tool and heirloom of the Jiang clan. It is a silver ring with a purple crystal and can turn into a whip. The name Zǐdiàn (紫电) literally means "purple lightning", referring to the purple colored lightning that the whip-form emits.
Jiang Cheng wears the ring on his right index finger.
Zidian's special ability allows it to separate the soul and physical form of an individual whose body has been seized by a spirit. It is guaranteed to succeed unless the soul has been forced into the physical body as in the case of Wei Wuxian, who was forced into Mo Xuanyu's body through a sacrificial ritual.
Zidian only responds to its owner or a person its owner has permitted to use it.
In Chapter 7 of the novel, it is stated that Jiang Cheng touching the ring is a dangerous sign of his killing intent. In Chapter 8, he contemplates using it against Lan Wangji who was opposing him, but ultimately decides against it. He does draw Zidian against Lan Wangji and his guqin, Wangji, in Chapter 10 and lands a hit on Wei Wuxian's back. Although Wei Wuxian's soul was not expelled, Jiang Cheng nevertheless continues to suspect it is him in Mo Xuanyu's body.
In Chapter 23, Jiang Cheng again strikes Wei Wuxian, this time on the leg as he was attempting to run away. He leaves Zidian with Jin Ling when he goes to investigate the supposed appearance of Wen Ning. Jin Ling entrusts Zidian with one of Jiang Cheng's subordinates before running off on his own to escape his uncle's wrath.