A layer I have no idea what to do with.

Had this idea, was thinking it might make a good site somewhere on the Outer Planes, haven't the foggiest where it might go though. I give it to you as... something. A god's domain? A layer of the Abyss? A section of Mechanus? A section of the modern, high-tech Mechanus? A large site in the third layer of Pandemonium? A secret test site in Belierin? An ethereal demiplane? Part of UnderSigil? ...too many ideas...

The Binding Leys

As your eyes adjust to the dimness, you realize that the tiny light in this vast space comes from inscribed runes that make overlapping circles and lines on the floor, big enough for a large creature to stand in and walk through. As your ears sift through the susurrations of the air, you realize that several of these circles contain creatures, few of whom seem pleased with their situation. And as you examine your surroundings, it suddenly dawns on you that your gate has let you out within one of these circles yourself...

This region consists of an enormous stone surface on which there are a large number of permanently inscribed spells of magic circle against evil, as well as law, and chaos. (Perhaps good as well, depending on its ultimate nature, though circles against law and chaos would also suffice to contain archons and eladrin, though not guardinals.)

The circles appear in many different styles: shining metal, dried blood, inlaid gems, letters incised in stone, powdered sand, arrays of quartz crystals and wires, and more. Each is of enormous size -- the typical magic circle spell would not be big enough -- and all are enhanced with calling diagrams to anchor their prey. In addition, some of them are oddly shaped, laid out as long straight or twisting tunnels. The surface on which they are writ is not flat: it rises, slopes, branches, and loops, forming a huge network of paths hanging in what appears to be darkened space. Many of the circles are linked. Many of them are currently inhabited.

A common case in the Leys is simple and ironic: a demon and a devil are both bound in a magic circle, one against law and the other against chaos. The two circles overlap slightly. Each could free the other. There are plenty of odder shapes: one may come across a pair of magic circles against chaos, one focused inward and a smaller one focused outward, the annulus between them containing a demon, and inside the smaller circle a yet smaller magic circle against law containing a devil. An entire array of active, interlinked magic circles against evil may require a devil to wander through prescribed passages, in a mazelike fashion, or surround and block off access to a gate, where a human could simply walk across.

Such measures are effective since the entire realm has volumes of forbiddance spells covering vast swaths of the place (and affecting all manner of alignments, some of which change without notice). They can be dispelled (the cast is apparently of rather high level), but eventually a given forbiddance will pop back in to existence. Whether there is a pattern to these things is not imediately clear: there never seems to be a caster around to cast the forbiddance, or to activate the circles or summon a creature into them. Sometimes spell effects will also simply cease, often letting two blood enemies who have been snarling at each other for days tear into one another. The survivor can then go off and hunt for a gate out of the place, of which there are several.

While a simple, prime-material human can walk across the diagrams with ease, safety is another matter. It's easy to accidentally disturb one if passing casually, thereby freeing the inhabitant. Plenty of the inhabitants of the circles have the ability to become invisible, and when a traveler is inside the circle they are free to attack, charm, or otherwise affect him. It is quite impossible to navigate the entire realm without ever crossing a circle, especially if one wishes to avoid counter-aligned forbiddance effects.
There are also the encounters one may have in the Leys: freed creatures seeking a gate, or creatures who freed themselves and now are wandering the Leys for some other purpose, such as to prey on travellers or learn its nature. There could always be the beings who made the Leys, pursuing their purposes.

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So, two big questions. Why would one wish to come here? And why would anyone build such a place? If a planewalker is of a nature that allows them to be summoned and bound as an extraplanar creature, they might find themselves trapped here, and having to work their way out, but anyone else is likely to either slip through a portal accidentally or come here looking for something. Perhaps this is a place where, inscribed among the diagrams, one may find the lost true names of many fiends. Perhaps they're here to rescue an archon who has becomed trapped in a magic circle against law (or good). Perhaps some spellcasters are here to research the behavior of extraplanar creatures, or summoning and binding spells, or related matters.

As for who built the place and why, answers could range from an obsessed Prime wizard's powerful ethereal demiplane to the arcanaloths to the guardinals... unless it somehow arose naturally. And there's always gods, who are about the only entities in the multiverse capable of running a long, complex demon-summoning project without the whole affair being likely to crash down onto their heads fatally. Annoying some demons and devils for a few days each (eventually they'll make the Will save to escape a given circle) is hardly worth the expense of time and magical energy it must have taken to create this place, so there's almost certainly something the builder wants.

Are the summoned creatures' energies being sucked away by the network to power some experiment deep within the leys? Is the point to harness energy from the clashes of beings of different alignment (which would mean that you'd have a much more potent reason to add magic circle against good to the mix, and throw celestials against fiends)? Is it to study how extreme alignments interact (maybe a project of the rilmani, then)? Or an attempt at examining the behavior of a large, complex, organized system -- the network itself -- faced with the chaos of conflict and chance Will saves (so a project of a lawful group examining tactics to press against universal entropy)? As part of UnderSigil, instead of summonings and worrying about beings teleporting out, you'd simply have portals all over the place, dumping exemplars into binding circles for a few days and letting them go or resupplying whenever they needed more -- a sort of alignment counterweight system to the city above.

Lots of options. Who can say? The Leys await your creativity.