The difference between an oracle and a necromanteion was the association of the latter with the Underworld; sanctuaries where necromancy was practised were most often dedicated to Hades and Persephone, the rulers of the Underworld, or to heroes who had died a miraculous death and were hence worshipped as gods. Described in literature as early as book 11 of the Odyssey, the relevant rite included a blood sacrifice, during which the deified heroes would drink of the blood and be able to remember their own past lives as mortals, as well as give oracles about the future.
As a mythological character, Amphiaraus belonged to the Theban Circle and was one of the Seven against Thebes in the Aeschylean version of the myth. Having been a famous seer gifted by Zeus and Apollo in his mortal life, he reluctantly led the campaign against the city, knowing it would result in his own demise. To protect him from dying in battle, Zeus split the earth open with a lightning and Amphiaraus descended into the Underworld along with his chariot, thus becoming a chthonic deity.