I would say the Shikon no Tama, but the jewel has two wills, that of Midoriko and of Magatsuhi, thus the jewel can be either good or evil depending on who has it, but the question only adresses the evil aspect of the jewel's deigns or be it it's will, and so I say Magatsuhi, a creature who seemed to have power of influence over Naraku himself, and preactially held him in its locked jaws throughout the eniter series. Least that my take on it anyway;]

Before even Magatsuhi, there was the shikon no tama

Be it the jewel or Magatsuhi I agree fully with its . . . unnerving influence over everyone.