Before Delphi was the most consulted oracle in the ancient Greek world, it was Python's territory — a great earth-born serpent guarding the site the Greeks called Pytho. Its death, at the hands of a barely-born god, is the founding act behind one of antiquity's most influential religious institutions.
Python was born of Gaia, the earth herself, and is closely tied in most tellings to the goddess Hera's jealousy of her husband Zeus's affair with the Titaness Leto. While Leto was pregnant with Apollo and Artemis, Hera sent Python to pursue and torment her, preventing her from finding any solid ground on which to give birth — Leto was ultimately forced to deliver on the floating island of Delos, the one place Python could not reach.
Once grown, Apollo — remarkably, according to several ancient sources, still only days old — set out to avenge his mother's suffering directly, tracking Python to its lair near Delphi.
Apollo killed Python with his arrows at the site later known as Delphi, and in the aftermath established his own oracle there, taking over the sanctuary the serpent had guarded. The site retained its old name, Pytho, and — in the serpent's honour — the oracle's own prophetic priestess became known ever after as the Pythia.
The etymology is genuinely uncertain: a popular ancient folk explanation claimed "Pytho" and "Pythia" derived from the Greek verb pythein, "to rot" — referring to Python's corpse left to decompose in the sun at the site. Modern linguists treat this connection with real scepticism; it may be a later, appealing folk etymology rather than the word's actual historical root, and this reference presents it as tradition rather than settled fact.
Even a god needed atonement: killing Python, monster or not, was still treated as bloodguilt requiring ritual purification. Apollo was said to have served a period of servitude — in some tellings eight years — to the mortal king Admetus of Pherae as expiation. The Pythian Games, held at Delphi every four years and second only to the Olympics in Panhellenic prestige, were partly understood as a continuing commemoration of the slaying and Apollo's atonement for it.