The endless void.
It was rare to encounter anyone in the astral plane.
However, Soran seemed to have encountered someone. Someone that was using his real body to travel.
There was no air or gravity in the astral plane. Projection was, of course, the simplest way; using a true body would require plenty of energy.
Indeed, Soran had encountered a Lich!
As an undead, many limitations of the astral plane did not affect it.
In fact, this was the first time Soran encountered a Lich.