Introduction
Ambarisha is a king of the Ikshvaku dynasty, Rama's own line, and one of the best-known royal devotees of Vishnu in the Puranic tradition. He is remembered chiefly for the story of his Ekadashi vow and his encounter with the sage Durvasa.
This article primarily follows the Valmiki Ramayana. Where a well-known detail comes from a later retelling or tradition, the article identifies it.
Note on sources
Ambarisha's story as a devotee of Vishnu is a Puranic one, told at length in the Bhagavata Purana, and is not part of Valmiki's narrative. He is placed in the Ikshvaku line as one of its kings; Puranic and epic king-lists differ on his exact position.
The Ekadashi vow and Durvasa
Ambarisha observed a strict vow of fasting on Ekadashi, the eleventh lunar day, and had arranged to break his fast at a precise, auspicious moment. As that moment approached, the irritable sage Durvasa arrived as a guest and asked Ambarisha to wait while he bathed before they ate together. To avoid the fault of breaking a vow, but also of failing his duty as a host, Ambarisha, on the advice of Brahmins present, sipped only a little water to end the fast without eating, and waited for Durvasa.
Durvasa returned, learned what had happened, and, enraged at what he took as an insult, created a fierce being from his matted hair to attack the king. Ambarisha did not defend himself, trusting in Vishnu. Vishnu's discus, the Sudarshana Chakra, destroyed the demon and then pursued Durvasa himself across the three worlds, since it protects any sincere devotee. Durvasa fled to Brahma and then to Shiva, and finally to Vishnu, who told him that the discus would not stop unless Ambarisha himself asked it to, since the offence had been against him. Durvasa returned to Ambarisha and begged forgiveness, and the king, humbled, asked the discus to spare the sage.
Meaning
The story is told as an illustration of the protection Vishnu gives a devotee, even one who has, without any intended offence, upset a powerful sage, and of the importance of humility on both sides.