Lightless, cold, and silent, the Underworld is the anathema of magi who master forces. There are all manner of living things here, but they are strange and difficult to fathom. There is no real creation of matter in the Underworld, merely stuff that is brought by various entities. The dark, chilliness of the Underworld is depressing, and visiting mages have been known to go off into Quiet. The Underworld is more loosely connected to Prime than the Dream- lands themselves.
Spirits are still quite easy to summon, but they are often malevolent or just strange.
Time does not shift detectably in the Underworld. This 250-page book contains a goldmine of information on the Dreamlands. Probably out of print, which is a damn shame. I'll expand more on this later thsi week, and add some concrete numbers for beings, descriptions of places, what the different factions of the Ascension War and the WoD think of it, do here, etc.