This type of dream is not very common. The name transpersonal derives from the so-called transpersonal experiences, which are characterized by powerful occasions in which the sense of corporeality is lost or our senses expand beyond our body.
In these moments the spirit merges with the unity of being, with the Universe.
It is usually a process in which personality dissolves, time and space disappear. The light begins to fill you and everything flows into a moment where you have to make a choice: to let yourself be carried or not by the experience that is presented.
This is a moment that can be frightening for anyone, for if you want to let yourself go, you will lose the usual sensation of your body. You will not feel your skin, your heartbeat, or your breathing in the same way, and this can be uncomfortable for some people.
Expansion and/or dissolution of the ego.
Psychedelics can often be portals to transpersonal experiences. These substances alter our habitual perception and depending on what you take, dissociation can easily lead to a transpersonal experience.
We may be presented with guardians, or some entity to guide us in this process, but the most relevant to the whole experience is the sensation that your being/feeling transcends your person, to a greater or lesser degree. Altered states of consciousness often include heightened perceptions that are, in fact, transpersonal; they are attuned to the surrounding energy.
If the sense of oneness encompasses your whole mind, you see only light and feel yourself floating weightlessly, it is called a peak experience.
When the experience occurs during our sleep this often translates into dream situations in which we feel that we die, that we cease to be us. Our identity on earth is no longer relevant and the consciousness expands feeling the eternal source of the Universe.