Apollo is a god of many domains, and his identity shifted across the Greek world: earliest a god associated with light, youth, and the archery, he came to be strongly identified with the sun (a Hellenistic development — the 'sun-god' Apollo is later than the archer Apollo) [S4][S5]. He is the god of music (lyre), poetry (the Muse-calling), prophecy (his oracle at Delphi, the Pythia), medicine (his son Asclepius), and archery [S4][S5][S6]. He is the twin brother of Artemis, and both are children of Zeus and Leto (Latona) [S4]. As a child he slew the Python that guarded the Delphic oracle, founding his sanctuary there [S4][S5]. He is also a god of plague (he sent the arrow-plague on the Greeks in the Iliad's opening) and of its healing — a duality the sources preserve [S2][S4].