The Mahabharata - pg. 16

Consumed with grief and rage, and kept alive only by the passion for revenge, Amba went to the Himalayas and practiced rigorous austerities to get the grace of Siva, now that all human aid had failed her. Siva appeared before her and granted her a boon, that in her next birth she would slay Bhishma.
Amba was impatient for that rebirth which would give her her heart's desire. She made a pyre and plunged into the fire - pouring out the flame in her heart into the scarcely hotter blaze of the pyre.