The spell Wall of Stone allows you to create stone walls and fortifications from thin air. At 60 hp per inch, the difficulty of getting out of it would add up fast.
Since the wall doesn't have to be a specific shape, it seems one could make a dome out of it. A quarter of a dome 10 feet high and 35 feet in radius (putting the target in a corner of a room) takes up 90% of the surface area available, with plenty to cover inefficiencies, and few creatures could get out from under it, even if they made their Dex save.
If the above wall were double thick (casting a second dome right over the first), this puts 360 hp of stone between the target and escape. In the extreme case, placing all 10 panels together to block off a dead end corridor is 1,800 hp, still with a 15 AC to hit (and is 5 feet thick). With a single spell. This effectively ends any attempts to escape inside combat, concentration presumed.
Is there a RAW reason that I cannot trap someone behind 5 feet of stone?
If the space is small enough, say, they failed the save on being trapped in their 5 ft square, is there a RAW reason suffocation would or wouldn't be a factor, or is it entirely the DM's discretion? There is some precedence,
in the Oozing Temple, one of the optional dungeons in Ch 2 of OOTA.
I realize Stone Shape, Passwall, Dispel Magic (to an extent), and Transmute Rock all work as magic counters.
Best Answer
Almost any creature could escape before suffocating
In most circumstances, the victim would have ample time to chisel their way out.
Wall of Stone creates 10 foot panels (or larger). So let’s assume: (1) your spellcaster trapped their victim in the minimum size, 10' cube, and (2) they concentrated on the spell for 10 minutes, making the stone permanent.
I’m not aware of any rules that would cover suffocation in these conditions, so let’s fall-back on real-world facts. This Friday Fiction Facts: Trapped in an airtight room! article calculates it would take hours]for a person to suffocate in those conditions
“A moderately active or stressed person” would have about 17 hours until they experience symptoms such as “panting, dizziness, severe headache, vision disturbances” at which point we will assume they can not longer effectively chisel at the wall.
At 30 HP/inch times 6 inches, the victim needs to do 180 HP of damage to the AC 15 wall to escape.
Let’s assume our victim is a human commoner with no strength bonus. We’ll even take away their club listed in their stat block, and just give them an improvised weapons, with which they get no attack bonus. Each round, they can do 1d4 points of damage to the wall if they “hit” with a roll of 15 or higher.
With those assumptions, the commoner’s average damage-per-round would be 0.75 HP. On average (and with this many “rolls,” most attempts would be very close to average) it would take 240 rounds to chisel out.
A round is 6 seconds; there are 600 rounds in an hour, so our mild-mannered commoner can chisel free in 0.4 hours, or 24 minutes.
So even if we make assumptions that our victim needs to rest three-quarters of the time they still get out with hours to spare.
If you are trapping an armed and dangerous creature, they are likely to be able to escape in even less time.
Edge cases
It’s easy to come up with edge cases where someone would not be able to dig out, or if the wall were made double thick, or more (which would require a lot of time since you have to concentrate for 10 minutes for each effect). In these cases the rules pretty much silent (the Portable Hole mentions death by suffocation in an enclosed space, but that’s a pretty different case).
Would there be tiny cracks in the wall, or the floor beneath it that allow air to seep in (like there are in the building where you sleep)? I think that is simply up to the DM. This is a world where you can breathe miles deep in the Underdark — the whole “how do we breathe” issue gets a little glossed over.