The crowned Buddha on a seven-headed serpent (naga) depicts a transcendent Buddha, in contrast to a historical Buddha. Distinctive to the Khmer kingdom, this image was especially important for King Jayavarman VII, who reigned 1181-1218, when tantric Buddhism was the state-sponsored religion. He considered the Naga-enthroned Buddha to be the ideal form of an enlightened being, and used the image to refer metaphorically to himself.
After the 13th century in Southeast Asia, images of the Naga-enthroned Buddha were appropriated by followers of a form of Buddhism that emphasized the life of the historical Buddha. The images then came to be understood as depictions of a scene just after the Buddha's own enlightenment when a serpent shielded him during a flood, so that he could continue his meditations without drowning.
Art Gallery of New South Wales, Sydney; 11/10/01-2/24/02. "Buddha: Radiant Awakening."
The Cleveland Museum of Art (05/05/2013 - 09/15/2013); "Focus:Tantra in Buddhist Art"