Here I'll collect all the information about the pocket regio level. "Grotto" might be an unfortunate choice of words due to other things in existence in the saga; I mean only "the basement level of the cave".
location of the entrance to the pocket regio
The pocket regio is accessed from the plateau on which the Tribunal city will be built, close enough to the center that it will be within the city bounds.
accessing this pocket regio
The maga holds her arms straight out at her sides like a cross, then shuffles sideways in a very particular spot. She will find herself inching through a cleft between two tall rock walls (that weren't there before). Several claustrophobic steps later, she will emerge from the fissure. The only place to go from inside the pocket regio is back out to the plateau in the main regio, by inching through the cleft in the other direction. (In particular, the pocket regio does not itself have a second level anywhere.)
appearance inside the pocket regio
Whereas the plateau "outside" was relatively flat and orderly, almost to the point of appearing manicured, here the terrain consists of scant trails winding between tall fang-like walls and ominous pits. ("It's like the deeper down you go, the more vertically stretched it gets. Like taffy.") From any given vantage point, one can see at most ten paces in any given direction: objects seem to be blurred and distorted as if seen through a haze of hot smoke, even though the air is uncolored and tastes normal; the sky is a featureless gray canvas consistent with a gloomy day. The haziness hindering visibility does not depend on the distance from the object to the viewer, but rather seems to be a constant feature of the regio's terrain. Details become harder and harder to see the farther they are from the crevice through which they entered; about 20 or 30 paces out, telling the difference between ground, pit, pillar, and sky is so hard that traveling further would be essentially suicidal.
The columns, walls, and even the ground are made of "junk rock", extremely crumbly and barely worth calling rock. There is a hint of a breeze, though mostly near the ground. Dirt and, occasionally, small pebbles are disturbed by the low gusts and disappear into the pits. There is a long but narrow crack in the ground somewhere in the regio, which a large person would have trouble fitting through. It leads to a lower cave, equally convoluted with stalagmites and unsure footing, with a thin coating of dim gray fungus on most surfaces.
magical contents of the pocket regio
The magical aura in the regio is weaker than the main regio: it has a score of 3, and is Perdo-aligned rather than Rego-aligned. This lowermost cave, below the narrow crack, is home to a magical cave salamander who has the power of invisibility. If physically trapped or held, he will drop his tail and run away; the tail contains a pawn of Perdo vis and takes a few months to grow back. The salamander's dung fertilizes the fungus in the cave, especially in a particular corner, where the fungus too is rendered invisible; this invisible fungus can be harvested for Imaginem vis.