Parvati created Chandi who in turn created Maa Kalaratri. It is at this point that the lore of Raktabija comes in. Raktabija was a demon who was granted the boon of every drop of his blood ...

The legend goes that the demon Raktabija, was blessed with the power that every drop of blood that will be spilled will birth a new demon. This was making it impossible for Bhairava to kill him.