Who is Ravana from Gods, Demons, and Others and what is their importance?

Ravana is a demon. Born with ten heads and twenty arms, he was originally known as Dasagreeva, but was given the name Ravana by Shiva, when he attempted to move Mount Kailas, Shiva's home. Ravana prays in penance for nine thousand years, and he is granted a boon by Brahma. He asks for immortality, naturally, as this is the main request of demons. Brahma refuses and gives him the gift of indestructibility instead. Every time one of his heads was severed, it would immediately grow back.