The story of Raktabija
This interesting story is available in Markendeya purana. According to the eighth chapter of Devi Mahatmya from Markandeya Puran, there was a demon by the name of Raktabija (blood-seed, literally).
Boon from Lord Brahma
He had been given a boon by Lord Brahma that every time a drop of his blood fell on the ground, his strength would increase many times, by his blood creating more of his own. Having received the boon, he had made the life of people on earth miserable.
He was,causing a great deal of trouble with people and gods alike but even worse was his ability to produce more demons every time a drop of his blood spilt to the ground.
Emergence of Kali and slaying of demon
So whenever he was attacked and harmed, more Raktabijas were born and fought the attacker.The gods decided to work together and combine all of their shakti or divine energy and produce one super being that could destroy Raktabija; the result was Kali.
Maa Kali’s most common pose in paintings is in her most fearsome guise as the slayer of demons, where she stands or dances with one foot on a collapsed Shiva and holds a severed head. She wears a skirt of severed human arms, a necklace of decapitated heads, and earrings of dead children, and she often has a terrifying expression with a lolling tongue which drips blood.
Kali in her fierce form eliminated the huge army and swallowed all of them, which left Raktabija all alone. She then struck the demon and before his blood spilled on the ground, she stretched her huge tongue and devoured all the blood ensuring that not a drop fell on the ground. This way, Raktabija was drained of all the blood and life and soon he was nothing but a corpse.