Raktbeej was a Raakshas who along with Shumbh and Nishumbh Asur fought with Durgaa. Raktbeej had got a Var (boon) that his every drop of blood which fell on the ground, will be turned into another Raktbeej. In fact that is why he was called Raktbeej - Rakt means blood and Beej means seed - because his blood worked as seed of another Raktbeej. Because he got multiplied from his every drop of blood which fell on the ground, that is why it was most difficult to kill him.

Devee Durgaa was in a flux when as she killed Raktbeej, his blood fell all over the ground and those drops were producing millions of more Raktbeej every time his blood fell on the ground. It had been impossible to kill him, so she produced her Kaal Raatri (also called Shubham-kaaree) form who licked all the blood from the ground and took every drop of his blood in her mouth before his blood fell on the ground and that is how she stopped his being duplicated from his blood drops. She killed him by licking every drop of his blood before it fell on the ground, because in this way neither his blood fell on the ground. nor any extra Raktbeej was born. This Devee is worshipped on the 6th day of Nava Raatri.

The 8th Chapter of Devee Mahaatmya describes his story along with the story Durgaa's battle with Shumbh and Nishumbh.

[Padm Puraan, 1/24]
[Skand Puraan, Maaheshwar Khand]