Astral conduits are corridors through the Astral Plane which go directly from the Prime Material Plane to one of the Outer Planes. They look like whirling funnels, except they’ve got no ends. No matter how far a sod travels, these conduits seem to go further. Conduits are dangerous, too, because they tend to thrash around and can suck up a traveler and spit him out someplace he don’t want to be. Luckily, the ends remain largely stable, most always dropping a berk off in pretty much the same place each time he passes through one.
The maw of a conduit is invisible to normal sight. Under a spell like true seeing, it looks like a reflective haze - a cloudy mirror is another way to describe it. The haze is opaque, and certainly nothing can be seen of where the conduit leads or what might be on the other side. Conduits have the great advantage of being quick. They pop a body right through the Astral Plane in a nod and a wink, so a cutter doesn’t have to waste any precious travel time.
As conduits are pretty well anchored, there’s a brisk trade in maps of their openings and endings. It pays to be real careful in buying these, however, because there’s more than a few cony-catchers out there with cunningly made forgeries, ready to work the cross-trade on some gully prime.
Note: Astral conduits originally appeared in this form in the Planescape AD&D campaign setting from TSR. Their use here is for the purposes of providing context for the campaign only.