Shri Krishna has a disc weapon, known as Sudarshana Chakra, in His right hand (usually depicted spinning around the right index finger). While the conch (Shank), held in the left hand, represents the lunar energy, the Chakra is associated with the Sun and the mind. The word Sudarshan means “good sight” or “auspicious to look at”.
According to Hindu mythology, Lord Vishnu wanted to obtain the weapon as a gift from Lord Shiva, in order to defeat the rakshasas (demons) that were troubling the world. Shri Shiva was deep meditation and Lord Vishnu did not want to disturb Him. He decided to please the ascetic Lord with an offering of 1000 lotuses, but when Lord Vishnu could only gather 999 lotuses He plucked out one of His own Lotus Eyes as a substitute. This great act of self-sacrifice greatly pleased Lord Shiva, and He gave Sudarshana Chakra to Shri Vishnu. With the invulnerable power of Sudarshana, the forces of evil were then defeated.
Shatrughna, the youngest brother of Lord Rama, and twin brother of Lakshmana, is said to have been an Incarnation of Sudarshana Chakra.
Source: Sahaja Yoga Australian Newsletter, 28 July 2006