Alchemy sets itself the task of acquiring this ‘treasure hard to attain’ and of producing it in visible form. — Carl Jung
The treasure which the hero fetches from the dark cavern is LIFE; it his himself, new-born from the dark maternal cave of the unconscious where he was stranded by the introversion or regression of libido.
Hence the Hindu fire-bringer is called Matarisvan, he who swells in the mother. The hero who clings to the mother is the DRAGON, and when he is reborn from the mother he becomes the conqueror of the dragon. He shares this paradoxical nature with the snake.
According to Philo the snake is the most spiritual of all creatures; it is of a fiery nature, and its swiftness is terrible. It has a long life and sloughs off old age with its skin. In actual fact the snake is a cold-blooded creature, unconscious and unrelated. It is both toxic and prophylactic, equally a symbol of the good and bad daemon (the Agathodaemon), of Christ and the Devil.
Among Gnostics it was regarded as an emblem of the brain-stem and spinal cord, as is consistent with its predominantly reflex psyche. It is an excellent symbol for the unconscious, perfectly expressing the latter’s sudden and unexpected manifestations, its painful and dangerous intervention in our affairs, and its frightening effects.
Taken purely as a psychologem the hero represents the positive, favorable action of the unconscious, while the dragon is its negative and unfavorable action-not birth, but a devouring; not a beneficial and constructive deed, but greedy retention and destructive. ~Carl Jung, Symbols of Transformation; Paragraph 560.
Mātariśvan (“growing in the mother”, from the locative of “mother”, mātari, and a root śvi “to grow, swell”) in the Rig-Veda is a name of Agni (the sacrificial fire, the “mother” in which it grows being the fire-stick), or of a divine being closely associated with Agni, a messenger of Vivasvat, bringing the hidden fire to the Bhrigus.
Sayana identifies him with Vayu, the wind, in Rig-Veda 1.93.6. In the Atharvaveda and later, the word also has the meaning of “air, wind, breeze”. It is also a name of Shiva, of a son of Garuda, and of a Rishi. ~Wikipedia
Note: In the quotation from “Symbols of Transformation, I have capitalized words which Dr. Jung had italicized.