Interesting situation came up in my session. The players captured a wizard NPC and put him in jail. Then came a long discussion about what measures needed to be taken in order to prevent said wizard from using spells to force his way out.
Basic measures like removing a spell book, holy/arcane focus, and spell components would prevent use of any spells with a material component. A gag would stop verbal spells and being bound would stop somatic spells. However this is an unsustainable level of treatment for a jail for more than a day or so since it would have to be applied to ALL prisoners since there is no easy way to tell who can cast and who can't.
Preventing a long rest would severely handicap most casters in that it would prevent recovery of spell slots/sorcery points, although a warlock wouldn't be affected. Seems like a basic precaution to not allow prisoners long periods of rest. But this doesn't affect unused spell slots and no character class, from what I can tell, "loses" spells if they don't perform some sort of ritual each day (they just can't change their prepared spells if applicable), even the wizard. So unless a caster is tapped out on spell slots/sorcery points before being jailed, they would always retain what they have left even if denied a long rest (and would still have cantrips).
So, apart from being tightly bound, gagged, and stripped, are there any ways to prevent casting that I'm missing?
There are plenty of verbal only spells that would be useful in a jail environment (Command, Knock, Misty Step) so really, how are casters supposed to be locked up other than some sort of dedicated magicked prison (assuming you could even transport them there)? Just taking out the gag to feed them opens up the possibility of a spell being cast. I don't want my characters to have to be treated like this if they get jailed, but it seems like in a world of scarse magic items but numerous magic wielding people, anyone not KNOWN to be non-magic using would be treated like Hannibal Lecter or put in an Arkham Asylum type specialty prison (bad example, folks escape from there all the time! Maybe the one from Harry Potter).
My homebrew solution is to have the higher levels of exhaustion stop casting. This way someone who has been in a jail for more than a few days can't cast due to poor conditions, poor food, etc. But they can still move around a bit and talk, so there are role playing possibilities (barter for more food, become prison kingpin to get better treatment, bribe a guard, etc). When captured maybe casters could be bound and gagged, but it wouldn't be necessary for long term imprisonment.
I'm using the basic vanilla forgotten realms setting with the suggested "low magic item availability" from the 5e PHB and DMG. Judging from the published campaigns I've read (Phandelin and Horde) there does not seem to be common availability of magical effects and devices. Party level is 5-9th level.
The answers to the previous setting-free question How could towns restrain a magic user? has plenty of useful "soft" suggestions like archers on the roof. But I would like some 5e and Forgotten Realms-specific techniques.
Best Answer
There are a number of different strategies one can take (besides the possibly obvious: remove material spell components/focuses/holy symbols/etc.):
Suppress sound (e.g., permanent or ongoing silence) to eliminate verbal spell components
Suppress light so that the spell caster cannot, for example, target individuals or locations via line-of-sight (e.g., use illusions over doors or windows to block line of sight).
Suppress divination (e.g., permanent or ongoing nondetection) so that the spell caster’s buddies cannot scry in order to pop in via, say, teleport, and rescue them.
The jail is built in an area that "naturally" suppresses, hampers or distorts magic.
The jail is constructed using permanent or ongoing anti-magic spells
NOTE: Following up on one of SevenSidedDie’s comments on the original question, spell caster-proof jails may be rare enough and expensive enough things, that one would not expect small towns, or even necessarily any particular city to have the capacity to hold a spell caster (although this will depend somewhat on how common spell casters are in the campaign setting, for example, if every small town has a few high level casters, then these kinds of jail enhancements might be common). This kind of scarcity might make the possession of a spell caster-proof jail a special resource for a region, or even between nations/principalities/city states/etc. where extradition treaties, rendition treaties, or even trade treaties are negotiated around arranging the incarceration of a spell caster (imagine a side narrative about arranging the transfer of a incarcerated spell caster: fun!).
Magically compel the spell caster not to use spells while incarcerated in jail (e.g., a geas, some really nasty custom curse, mark of justice, etc.).
Make the spell caster wear some difficult to remove (probably homebrew) magic item that suppresses somatic movements, sounds, or even magic.
Tag the incarcerated caster with difficult to remove tracking and summoning magic, and make them aware that there will be sanctions for jailbreaks.
Place the spell caster in some kind of suspended animation (e.g., the
vanishsink spell of 1st edition AD&D).Drug the spell caster so that they are incapable of casting (per orlp’s excellent comment).
Abjurationist wardens
Mage-slayer wardens (perhaps equipped for and specializing in non-lethal combat and restraint)
Wardens equipped with anti-magic devices
Wardens that are magic-suppressing, magic resistant, or magic immune monsters or constructs.
Threaten the spell caster’s familiar, family, colleagues, or benefactor with sanctions (e.g., your family pays a huge fine if you jail break; your familiar will be destroyed if you jail break; your teacher will be stripped of standing in the college of magic if vouchsafed spell casting students jail break; your first born child will receive the spell caster’s sentence if they jail break; etc.).
Appeal to the sanctity of law (if that is something that the spell caster cares about).
Escalate the legal threats against the spell caster by compounding an original crime with a jail break (e.g., the punishment originally was 6 weeks in the clink and a small fine, now it is the amputation of your right hand).
Make plea deals, so that publicly the spell caster is seen as punished, but the powers that be arrange to trade benefit for leniency in actual sentencing.