OK, nothing particularly interesting happens during the night.
Scott
Gregorius will head down to the regio entrance the next morning, with Alcimus, Fray and any shield grog Fray wants to bring (if asked, Gregorius will suggest Aison (https://forum.atlas-games.com/t/covenfolk-character-sheets/5242/1), who Gregorius sometimes uses himself in situations where a monstrous wolf would be inappropriate).
He'll look for the regio boundary, and try to assess the conditions on the other side (and briefly report them to Fray). Then he'll suggest:
"I propose that Alcimus and I go through first, and then return immediately for you assuming we're able to. If we're not able to" he smiles "you may wish to consider yourself lucky. Viola also has Second Sight, as do at least one of those cats, so there should be other people able to relocate the regio boundary if necessary. I'll take an arcane connection from this side, but I have no idea whether spells to it will work across the regio boundary, and unfortunately I never learnt an arcane connection range communication spell. Still, better to have it than not."
He'll chip a small piece off one of his stones, keep it and leave the stone in the corridor.
What Gregorius sees immediately inside the regio is a tunel, much like the one on the mundane level: a passage partly hewn by tools, but following the course of a stream. The main difference is that the water still flows here.
Scott
But there's light there now? It was "rapidly fading" the previous time.
Yes. It's coming from somewhere up ahead.
Scott
Gregorius will wait for Fray's response, then if he has no objections to the proposed plan step through the regio boundary holding on to Alcimus to lead him through as he does so. Then he'll try to return the way he just came.
Regardless of them returning, Fray will mark the floor with chalk where the regio boundary is, including "regio boundary" in latin so there is no confusion. He'll wait a good 15 or so minutes calmly.
Passing time by trying to talk to Aison about when and how often they train martial skills, as he as an interest in keeping his skills. As I assume she is in no way interested in a discussion with a strange magus, Fray will enjoy the waiting in slightly uncomfortable silence.
[OOC: Pres + Folk Ken total is -1 without the Gift penalty, so I'm not expecting any brilliance while trying to make idle banter.]
Gregorius returns quite quickly, leaving little time for awkward conversation with Aison.
Scott
"If we assume the regio is permanent and there is no significant dilation of time, then the breadth of this investigation just became more interesting."
"What did you see of the rest of the corridor when you looked at the boundary from the inside? "
"and lets explore the current mundane tunnel" and assuming he agrees Fray will walk further past the regio entry.
[I think we've already explored the mundane side of the tunnel? It led down to that empty pool. Aison is a "he".]
What did Gregorius see when he turned round in the regio? Did the tunnel continue back the other way as well, or was there just a way down to the pool?
Gregorius will lead Fray and company into the regio.
When he turned around, Gregorius only saw the tunnel he came from, at the mundane level.
When the magi enter the regio again and press on down the tunnel, they find that it opens into a large cave...and beyond the mouth of that cave lies daylight. The cave itself is unremarkable, and largely empty, but it looks as though someone at some point set up camp here, as there are a few discarded items strewn about, and the remains of a campfire.
Scott
Gregorius will examine the items strewn about (anything particularly interesting, or just standard camping remains?), then check the aura and cast Tale of the Ashes on the fire.
[Non-fatiguing aura spont. He's actually expecting it to be something other than 5 this time!
Tales of the Ashes is InIg 5 - a core book spell. Let's you know what the ashes were, and when and how they were burnt. Casting total is (In 6 + Ig 0 + Sta 1 +Aura >5 + die 4)/2 > 8, so should work. (invisiblecastle.com/roller/view/4486727/) He's not expecting anything beyond "wood, around the time the previous covenant existed", but it doesn't hurt to be sure.]
Then he'll go to the cave mouth and look out, trying to stay close to the wall so he's not as obvious to anything outside.
The debris is nothing of real interest--just a few items (tools and the like) that seem to have been discarded because they were broken. The fire was of ordinary wood. The one interesting thing is when it burned: it appears to have been lit on a number of occasions, but the most recent one was about three years ago.
Magical 7. 
As Gregorius approaches the other end of the cave, he's no longer sure that the light coming in is daylight: it doesn't seem bright enough. On reaching the entrance, he looks out into a large cave--perhaps half a mile across--filled with a lake. The light comes from the cave roof: something up there is throwing it off, quite brightly, from hundreds of tiny spots. It's not as bright as daylight, but it rivals the average Hermetic lab, and it's apparently bright enough to support a few spindly trees growing on a small rock island in the middle of the lake. Gregorius finds himself standing on a ledge from which the stream falls into the water, a couple of paces below him. The ledge extends around much of the cave wall, leading to entrances to additional passages, many with their own streams and waterfalls.
Scott
Mmm, that's a nice aura.
Gregorius looks surprised when he learns the fire's age.
"The last fire was here only about 3 years ago. Either this regio has some sort of time dilation, or there have been inhabitants since the fall of the last covenant."
I assume there were no obvious tracks in the area? Alcimus will have a bit of a look around, although it's probably been partially spoilt by Gregorius examining the strewn about items. Still, if there's any sign of anything that doesn't look human, that's worth knowing.
He'll peer up and around the greater cavern, somewhat impressed. Does the ledge look like it'll be relatively easy to walk along, or is it a "if you're not very careful you'll slip and plummet to your watery doom" affair? Are there any signs of any kind of life beyond plants?
"I wonder why the light was dimming when I saw it last night...What do you think? Should we try to explore the central cavern first, or the ledges? This does looks like it might be an excellent time to be able to fly."
The floor is mostly rock, so no obvious tracks.
Yes, you can walk along the ledge relatively easily in most places, though you will want to pay attention.
As for the light...Gregorius assumed it was dimming, but he wasn't actually looking at it long enough to know if it was actually changing.
Scott
Fray stands in awe standing near the edge of the shelf, looking out into chamber. "This is truly impressive". He clearly needs to take a moment to process the scene before him. He speaks softly to himself, "but why here?"
"Perhaps the light source is cyclic, akin to a day night cycle? For the tree to grow properly on that island it will need darkness as well as light. We need our rest, trees and plants need the same too I suspect."
"Lets make sure nothing untoward is around this entrance way" as he glances to the edges, "and then I'll fly to navigate the chamber, giving it a once over. I can look to the other chambers and see if anything pertinent or hostile is within them." Pausing, to shake his head at what the moment has brought. "Then as it might prove challenging to scale the waterfall, we can devise a safe staged plan for how we check each detail of the chamber. For now we know too little to really decide. "
"You too can fly if you wish, but I understand if you are wary. There may too be a trick to getting the effect to end without a token.".
Unless Gregorius objects or adds anything more, Fray will cast Cloak of Black Feathers (form CT +46, +aura, MuCo30), then Return of the Master's Voice (form CT +35, +aura, MuAn20).
When flying Fray will be cautious with the range to everything, at least out of bow range until he's sure things are not hostile. He'll do a few sweeps, checking each alcove, and then the small island. If there are signs of habitation on any of them that might change the plan.
Fray sees an occasional lizard, and signs of life but no indication that anything big lives here, like humans.
The island turns out to be pretty sizeable, and glimpses of movement indicate life there as well. Fray also thinks he sees something man-made on the island.
Scott
Fray will approach the island and see what he can make out in more detail. Not landing just yet, but taking a closer look; particularly the potentially man made thing. He's still planning on returning to the others to check in before walking around down there.
[Do you want a Hunt (tracking) or Awareness (search) check? for tracks, signs, etc]
Approaching closer, Fray detects nothing dangerous. He does see that the island is covered by a surprising variety of life: lizards, frogs, and even insects, and smaller plants in addition to the trees.
The manmade thing appears to be some kind of monument of altar, made out of the same limestone that everything else here (including the caves themselves) is made from. Fray can't tell much more than that from the air.
Scott
Fray will circle back to the platform with the others, brief them.
"I assuming you both will take a closer look, the matter is how to get you both down. I could perhaps increase my bird's size, which could be enough to carry you individually? Or perhaps climbing down."