Raavanan, Kumbhakarnan, and Vibhishanan prayed for several years to obtain a boon from Brahma. Raavanan wished for immortality but was denied, hence he requested that he shouldn't be killed by yakshans, ratchasans, serpents, birds, etc (excluding humans). The boon was granted. While Vibhishanan gets his wish granted as well, Kumbhakarnan (as a result of a trick played by Goddess Saraswati) gets the boon of being asleep his entire lifetime.
