So when we talk about the journey to the underworld, the metaphor indicates not a literal change of place: the individual remains in the same life they have been living already. Rather, it means a transformation in the way that individual lives, in the sense that their perceptions, emotions, thoughts, and behavior change. And although the details can vary greatly, the general direction is one where soul comes more to the foreground and the center, and ordinary life and the external world (including other people and our relationships with them) sink into the background.
A typical dynamic where this happens is what we call the “midlife transition”; and as Stein has pointed out, a central characteristic of that transition compared with other transitional phases during our lives is that it includes a journey to the underworld. Stein uses the Odyssee throughout his book, but rather than repeating motifs from the metaphorical narrative, let’s focus on the psychological interpretation.
The important characteristic is that the “journey” is not a literal change of place (in the space-time continuum). Its directional qualifier (“to Hades”) indicates what figurative meaning is intended instead:
The mythic realm of Hades […] describes a region of psychological existence, removed from the domain of standard social and psychological structures.
Murray Stein, In Midlife, 116.
Thus what the “journey” there amounts to is a gradual change in an individual’s relationship with the external world:
[…] as the sense of psychic reality increases, the sense of the material world’s importance declines. […] Consensual social reality, too, loses its power to convince, and the rigor of social role expectations to govern ideals and behavior goes flaccid. […] Experience of the world of inner objects becomes more important, and the relationships between these inner objects — the imagos, the figures of persons and the textures of places remembered and imagined, the faces and drama of the complexes and archetypes — assumes a degree of fascination and importance above the concern for interpersonal relationships or the demands from an external adaptive context. (Ibd. 120)
Once more, the change is gradual and directed (and it can be reversed for a “return” trip), but it doesn’t have to be directed at a specific goal or end state. In any case it is a turning inwards, and thus a change in personality — the same which Jung has already described in Symbols and in Psychology & Alchemy, and found to be both necessary in certain dynamics and also dangerous.