Edward Edinger gives a long description of this picture in The Mysterium Lectures (pp. 132-137). While I recommend people to read this part in its entirety, I will quote the essential elements given the context of this article.
Edinger argues that The Pandora represents in a single image the “total Christian Weltanschauung” (ibid.). This means that it represents Christianity compensated by alchemy: The Pandora makes visible the union of both traditions.
Surrounded by the four evangelists (John as an eagle, Mark as a lion, Luke as an ox, Matthew as an angel), we see in the upper half the crowning of Mary by the trinitarian God (the Father, the Son, and the Holy Spirit). So far so good.
Looking at the lower half, we see the ongoing alchemical work. A haloed figure is extracting a monstrous figure out of a lump of matter. Quoted by Edinger, Jung comments:
“Underneath the coronation scene [is] a kind of shield between the emblems of Matthew and Luke, on which is depicted the extraction of Mercurius from the prima materia. The extracted spirit appears in monstrous form: the head is surrounded by a halo, and reminds us of the traditional head of Christ, but the arms are snakes and the lower half of the body resembles a stylized fish tail. This is without doubt the anima mundi who has been freed from the shackles of matter, the filius macrocosmi [son of the great world] or Mercurius-Anthropos, who, because of his double nature, is not only spiritual and physical but unites in himself the morally highest and lowest. The illustration in Pandora points to the great secret which the alchemists dimly felt was implicit in the Assumption. The proverbial darkness of sublunary matter has always been associated with the “prince of this world,” the devil.” (Carl Jung, Mysterium Coniunctionis, CW 14, par 238)
By virtue of being identified with the Anthropos, this monstrous creature is the emergence of the self, not as a symbol but as a living experience.
“Symbolic images of the Self, such as this quaternity-creating one of the Coronation of the Virgin Mary and her entrance into heaven, are beautiful, grand and numinous. But the living experience of the Self is very different indeed. The living experience of the Self is a monstrosity. It’s a coming together of opposites that appalls the ego and exposes it to anguish, demoralization and violation of all reasonable considerations. That’s what a monstrosity is—a violation of everything we’ve come to expect as natural and reasonable and normal. This is how the Coronation of the Virgin in heaven looks from the standpoint of the limited, earthbound ego—it looks like the emergence of a monstrosity out of a lump.” (Edward Edinger, The Mysterium Lectures)
By crowning Mary, the God-image is being transformed to include the unconscious. This can be understood as the addition of the feminine, of matter, and of evil into the Godhead.
“[T]he center of the Christian mandala is like a cuckoo’s egg that’s been laid in somebody else’s nest. It’s been laid in the nest of the Christian mandala and something unexpected is going to hatch out of it! I think it signifies that the central myth and God-image of the Western psyche, by virtue of this alchemical process that’s been inserted into it, is giving birth to a new entity.” (ibid.)
To birth this “new entity”, the ego has to go through the gruelling alchemical transformation of being turned inside-out. Edinger describes the result of this process in the following manner:
“Now out of this lump a bizarre creature is being pulled by a crowned and haloed figure. What should we understand this figure to represent? My suggestion is that we might call it the Christified ego. What I mean by that is an ego functioning under the aegis of the Self, the crown and halo being symbols of the Self, of wholeness. And if that line of thought is right, then the ego is doing on earth what Christ is doing in heaven. Christ in heaven—in the archetypal realm—is crowning the Virgin Mary; there, the principle of materiality and egohood is being glorified. But on earth, the task of realizing that glorification is taking place through the redemption and transformation of concrete personal existence by the individuating ego; in other words, by an ego that’s consciously living out the process of continuing incarnation.” (ibid.)