Nobody else agreed to bring the mother of the witches to Earth; Orisha Nlá made them swear on the Ground not to destroy human lives without cause: that is why the witches have no power against the true children of God or the devoted of Orunmila.
Against the power of witchcraft there is only one force able to beat it: the ground, the earth. So God proclaimed, setting the ground as the only power meant to destroy any force or divinity that broke one of the Natural Laws. The decree was issued when a powerful witch from Heaven, named Eye To Yu Oke To Ku Orun, was found to be involved in the destruction of the earthly divinities, because of her wicked conduct on Earth. The one who brought her into the world, she being the mother of the witches, was Oshé Sá (Osemolura), because no one else agreed to do it; and the oath that Orisha Nlá made the witches swear was the Ground.
That oath was sworn against the destruction of human lives without justification. That is where it comes from that the witches have no power to finish off the true children of God, and they have no power either against those who follow Orunmila with devotion.
As for the power of the Elders of the Night — the witches —, Almighty God gave it to them in that time when he lived free and in body among the divinities. It fell to them, and only to them, to stand guard while God bathed, right before the rooster crowed, because it was always forbidden to see him naked. Only the witch divinity was granted that privilege, and sometimes it was she who told the rooster that the bath was over.
Notes: through Oshé Sá it is known in what way God protects us from the powers of witchcraft. The witches destroy whoever deserves it: when men and witches treated each other as equals, the men allowed the witch's son to be eaten, and from then on the witches ruled to finish off the children of the earth.