The island of Nagadeepa lying about 24 km away from the mainland was once inhabited with people belonging to Naga clan.

According to the chronicles, a great battle erupted between two Naga kings, Chulodara and Mahodara over the claim of a gem-studded throne. Chulodara is the son of Mahodara's sister and the battle developed after the gem throne was given to Chulodara with the demise of the father of Mahodara who wanted to have the throne in his possession.

God Sumana, a protective deity reached Lord Buddha at Jetavanarama in Sravasti to inform of this war to break out soon. Lord Buddha arrived in the island of the Nagas in the 5th year after enlightenment, preached the virtues of non-violence to the two kings and settled the dispute over the gem throne between the uncle and the nephew who offered the gem-studded throne to Lord Buddha.

Later, that throne was enshrined in a stupa for Buddhists’ veneration. The arrival of Lord Buddha occurred on the Bak (April) Full Moon Poya day. Since then Buddhists gather to this sacred island to commemorate Lord Buddha's arrival. Veteran artist Solius Mendis, in his paintings at the Kelaniya Temple, describes how the Nagas became followers of Lord Buddha.

According to legend, God Sumana was a resident of Nagadeepa in his previous birth and had offered some leaves from a tree to Pachcheka Buddhas to clean their alms bowls. This merit made him a deity in his next birth.