Ambarisha was the son of King Mandhata and brother of Muchukund. He was an ancestor of Shri Ram.
He was a great devotee of Bhagwaan Vishnu and Narayan had given him his Sudarshan Chakra for his devotion, which he worshipped with great devotion.
Ambarisha started the Ekadashi (11th day of fortnight) fast, which is observed for Bhagwaan Vishnu and the fast is broken at the start of Dvadashi (12th day of fortnight).
Once sage Durvasa was visiting. It was time for Ambarisha to break his fast and rishi Durvasha had gone to bathe in river Yamuna and was taking time. So Ambarisha broke his fast by drinking some water.
When rishi Durvasa came back, he took Ambarisha breaking his fast before him as an insult and in his rage created a demon to kill Ambarisha.
Because Ambarish was a great devotee of Bhagwaan Vishnu, Sudarshan Chakra intervened and killed the demon. Sudarshan chakra came after rishi Durvasa after killing the demon.
Durvasa rishi went to Brahmadev and Shivaji for protection and both refused to intervene. So, he went to Vishnuji and pleaded for his safety from Sudashran, which was chasing him.
Vishnu bhagwaan asked Durvasha rishi to seek pardon from Ambarisha. Durvasa did as suggested and Ambarisha pardoned him, thus saving his life.