He was an enterprising demon who disguised himself as a God during the great churning and inventing of Amrita and SOMA. He managed to get a good slurp of one of the immortalizing liquids before SURYA and SOMA shouted ‘Oi — you with the claws — what you doing with our cup?’
SHIVA, forever on demon alert, sliced off RAHU’S head, still with liquid in it’s mouth. The body fell to the underworld, as he had not had time to swallow, but the head was already immortal.
RAHU’s head flew up into the sky trailing blood and became a comet, forever grabbing at the Sun and Moon and causing eclipses.