My friend told me that Lord Rama's brother Shatrughna was an avatar of Lord Vishnu's Sudarshan Chakra. Is this real?
Quoting from this chapter of the Padma Purana where Lord Shiva is describing about the various avataras of Lord Vishnu to Goddess Parvati:
Describing about Sri Ram’s incarnation Lord Mahadeva told Parvati–‘Once, Swayambhuva Manu had expressed his desire of having Lord Vishnu as his son to which Lord Vishnu had agreed....
Continuing with the tale of Sri Ram, Lord Mahadeva told Parvati–Dasharath requested Vashishth to perform the Jaatkarm Sanskar of Sri Ram. In course of time Kaikeyi gave birth to Bharat–an incarnation of Panchajanya conch. Similarly, Sumitra gave birth to two sons–Laxman and Shatrughan. Laxman was the incarnation of Sheshnag while Shatrughan was the incarnation of Sudarshan Chakra.