Reahu is found in ancient Sanskrit mythology. The tale begins long ago in a land far far away, when the Gods and Asuras churned the Milk Ocean to extract the elixir of immortality. Reahu was there disguised as a God when the female avatar of Vishnu, started distributing the elixir of immortality to the Gods. While the Gods are lining up to receive the elixir, the Sun God and the Moon God noticed Reahu in disguise and they informed Vishnu. Realizing Reahu is an imposter, Vishnu draw his sword and chop off Reahu's head while he is drinking the elixir. By then it's already too late, Reahu's head is now immortal but his body is not. Now Reahu's head is separated from his body and vow to take revenge on the Sun God and the Moon God for their misdeed. Reahu would chase the Sun and Moon, catching and eating them. Since he doesn't have a body, the Sun and Moon would only go through his mouth and come back out. Today, we call this act as Solar and Lunar eclipse. So when there is an eclipse happening, Reahu is doing his thing.