Biju

Three biju are currently known, Shukaku, Nibi and Kyubi. Each of the biju names are represented by the number of tails they have. Usually kept secret,l the info can't be found, but we know that they were fought over in the Great war. But they're power was uncontrolable so no one could kill them. So the thing they did was to seal each one in a new born child of the year of the great war. But now the akatsuki have found a way to take the chakra of thses demon's and seal them.