The First Pythian Games

This coin from Emperor Valerian was minted in his first year of reign. The portrait stype is much better than average for this emperor.

Valerian I, AD 253-260, AR Antoninianus, Rome mint, 1st emission, AD 253

Obv: IMP C P LIC VALERIANVS AVG, radiate, draped, and cuirassed bust right

Rev: APOLINI PROPVG, Apollo standing right, drawing bow

Ref: RIC V 74

The reverse "to Apollo who defends us" references the story of Apollo killing the serpent at Delphi and creating the Pythian games, described in Ovid's Metamorphoses:

Unwilling she [Earth] created thus enormous Python. Thou unheard of serpent spread so far athwart the side of a vast mountain, didst fill with fear the race of new created man.
The God that bears the bow (a weapon used till then only to hunt the deer and agile goat) destroyed the monster with a myriad darts, and almost emptied all his quiver, till envenomed gore oozed forth from livid wounds.
Lest in a dark oblivion time should hide the fame of this achievement, sacred sports he instituted, from the Python called “The Pythian Games.”
- Ovid, Metamorphoses, The First Pythian Games, translated by Brookes More, 1922​

There is more to the story here on "Pythian" coming not from Python but from from Greek "putho" meaning "rot" of the serpent's remains and vapors from its decomposition inducing sybyl's prophetic trance. The story is illustrated in this 1589 engraving by Hendrik Goltzius which can be found in the British Museum collections.