The Asura (demon) Rahuketu was present at the primordial churning of the ocean of milk by the Devas (gods) and the Asuras to extract Amritam (the elixir of immortality). Passing as a Deva, Rahuketu drank the elixir and attained immortality but at that moment Vishnu, in the female form of Mohini, realized his identity and severed his head from his body. The head is known as Rahu and the body, Ketu. They are two of the nine major astronomical bodies (navagraha) in Hindu/Buddhist astronomy and astrology. Unlike the other seven, however, they are not planets but nodes of intersection of the paths of the Sun and the Moon.
In the Upakkilesa Suttaṃ of the Pali cannon, it is asserted that there are 4 defilements in the physical world that cause the Sun (Suryian) not to illuminate, not to shine and not to radiate - 'Storm Clouds', 'Fog' and 'Smoke and Dust', and 'Eclipses'. Phra Rahu is the anthropomorphization of this phenomenon of cyclic eclipsing or 'defilement' and the Suriya Sutta is the Suryian's prayer to Lord Buddha for protection. Upon its recitation, the Blessed Buddha appears and compels Rahu to release Suryian, ending the eclipse.
Phra Rahu is seen in Thailand as the essence of darkness and the harbinger of ill fortune. As such, he is placated with various black food offerings that mitigate his maleficence.These offerings include black herbal jelly .