Anti-magic prison in an iceberg

Question

This is in the context of D&D 5e. Although this could be more suited for that SE, I figured the magical engineers on here would be best suited to tackle this question.

In my homebrew world, up in the frozen, northern expanses, there is a prison specifically designed to hold dangerous magic users, with powers that could easily overpower a normal prison setup. Although magical bindings exist that can repress people's magical abilities, they are quite expensive and have been deemed inhumane to use long term, so this prison now uses a blanket anti-magic field to keep the inmates tame. The prisoners are transported in wearing anti-magic shackles, but after being placed in a cell these are removed. These shackles act as extensions of the global anti-magic field, so that the shackles aren't affected by the field.

The biggest danger that this prison faces is the anti-magic field failing, either by tampering or by a defect (though this is incredibly unlikely). A failsafe would need to be installed to insure that any major interruption of the anti-magic field would result in all of the prisoners being killed (or similarly incapacitated until the guard could re-enable the anti-magic field, but probably just killed). My first thought was to install the prison inside an iceberg, which would be held by magical means above the surface, and if the anti-magic fields comes to fail, the iceberg would plunge into the water, drowning all of the inmates. However, this requires magic to be active through the anti-magic field, and then stop working once that shuts down.

So my question is this: what arrangement could act as a failsafe for the anti-magic field being deactivated, killing all the inmates? (The guards have sworn an oath of allegiance and have accepted that they will most likely die too in this event, but if we can keep them alive, that's a plus.)

Some more details regarding the question:

• the prison being inside an iceberg isn't required, but I think it would be the most practical way of having immediate danger surrounding the prison in the frozen north, and it sounds cool.
• although the prisoners would in theory regain access to their powers briefly after the anti-magic field fails, we can assume that any spell potent enough to get them out of the situation immediately (teleportation, gate, fly, etc) would require too much preparation or components that they would not have access to (except in the case of a convoluted prison break setup), and there are also mundane physical barriers to tangle with too (bars, cells, thick metal doors etc)
• the incapacitation or killing of the inmates needs to be swift, as leaving them free for more than a minute would give them too much of a chance of escaping. For example, dumping all the prisoners into another dimension would not be sufficient as they could in theory work their way back onto the material plane and take revenge on those who imprisoned them.
• the answer should follow the general philosophy and rules of magic in D&D 5e, though they can be bent somewhat to allow neat tricks. Large scale uses of spells such as Power Word: Kill would therefore be quite impractical, and semi-random effects like Fireball or Firestorm would probably be too erratic to ensure that all of the inmates are killed. It would probably be wiser to employ environmental methods to ensure that the whole complex goes down, but I'll leave that to you.
• if at all relevant, the world is of ancient Greek inspiration, and divine magic and influence are slightly more present than in classical D&D 5e. Arcane magic is also slightly more potent.

Answer 1

Solve the magic with more magic!

OK, so you need a contingency if an anti-magic field fails. It has to be activated immediately. Well, how about you inverse this relationship - it's the anti-magic field that suppresses the measure itself. Since the field neutralises magic, there is your answer what the nature of the suppression is: It's Magic! (adding a k at the end is optional)

As for what the suppression does, you have a lot of options. I'll try to give some ideas:

Non-lethal

A powerful binding spell can be placed on the prisoners. For example, a geas that will compel them to be calm and return to their holding areas. With the anti-magic field active, they have freedom to walk around and don't have to obey the magic compel. However, they don't have magic of their own nor the means to freely exit the prison. So, they can just roam the inside.

A variation is a powerful sleep spell placed on the prison. Think "sleeping beauty". The anti-magic field allows prisoners to spend their days awake. Stopping the field will make everybody enter a deep slumber until the field is enabled again.

Lethal

You can have some sort of lethal effect active at all times. An ever burning fire, a magic vortex, or the equivalent of a bomb implanted in the heads of prisoners. The anti-magic field keeps this disabled and the prisoners alive.

Mixed approach?

You can have magical enables adversaries alongside the prisoners. Maybe demons, some sort of constructs, or whatever magical beast you prefer. The anti-magic field also disables the magic of these beings. Should it fail, then the prisoners would be locked alongside something that suddenly became very dangerous. This is not perfect solution but can serve as a deterrent from prisoners trying to fiddle with that field.

What happens after the magic field is disabled depends on what sort of beings are there. Demons would likely go for carnage, although other beings might just be compelled to attack and incapacitate magic users, rather than outright try to kill them.

Answer 2

Misdirection - it's actually a delayed magic field.

Your prison has signs up saying "Caution: Magic suppression field! All magic is cancelled out". However that's not what's happening.

Your prison isn't in a field that suppresses magic. It's in a field which defers magic until after the field is turned off. When the field fails or turns off, all spells that have been cast over the history of the field take effect as if they were cast sequentially. (including mana drain).

When your prisoner goes into the cell, the a guard secretly casts "Instant Death" and then "Teleport x km north and Y km east" such that you end up in an infirmary.

The effects of these spells are delayed until after the field is turned off or fails.

If the prisoners sentence elapses, or they're found innocent and released, a mage casts "Undo recent death" on the empty prison infirmary operating table (or a medic stands there with a defibrillator), and then the field is disabled in their cell. In an instant - they die, are transported to the infirmary, and are revived at 1HP.

If there's a great escape, as soon as the field goes down, all the escaping prisoners suddenly disappear and found dead within convenient gurney distance to the morgue - unless they had already escaped from their cell in which case their dead body is now in a random location in the facility - unlikely to be found by their co-conspirators during the critical minutes of an escape.

If a prisoner figures it out, they can't cast "make invincible" as the death already applied, they can't cast "revive this corpse" as they wont be able to target the corpse as its about to move, and if they cast something really horrible and powerful it'll fizzle out as they've just been killed so have minimal manna.

Answer 3

Every prisoner gets tattooed with a 9th level Glyph of Warding spell, containing "Imprisonment: Slumber" or "Imprisonment: Minimus Containment" in it, with the condition to go off "When the anti-magic field fails."

As long as the prisoner is inside the anti-magic field, they are perfectly safe and the glyphs can't activate.

As soon as the anti-magic field fails, all the glyphs fire, and all the prisoners go to sleep. All the guards survive, naturally. There is no chance for the prisoners to resist this: their best chance would have been trying to cast Counterspell, but they don't get a reaction to "Minimus Containment" because it was cast previously. Nor can they counterspell the Glyph, because that's also already been cast.

Adding a new prisoner will require services of a very powerful wizard, but since you're already maintaining an enormous anti-magic field, that should not be a problem.

Answer 4

Infect them. With magical monsters and ailments, curses and hexes. Most worlds of this type have some dire magical variation of the Black Death kicking around somewhere. The moment the field fails, the inmates' diseases once again take effect. Don't forget to honor the banshees as backup singers for the prison opera, nor to send along the pet cockatrice as the prison mascot! In theory, this wouldn't need to include anything directly/irreversibly lethal to inmates or guards.

Answer 5

What do engineers do when they need a critical piece of equipment to continue working on failure?

Redundancy.

Why have a single anti-magic field generator when you can have four or five at different locations within your glacier. If any one of them fails, the rest keep chugging along. In fact, one field should be sufficient to prevent escape, and so any attacker would have to simultaneously disable all field generators at the same time! This is also 100% compatible with @VLAZ's answer as well: you can have a second failsafe that activates when the anti-magic field is lots. This too can have its own redundant systems.

If running all 4 consume too much power it's also fine to keep two active and keep the other two supressed and using VLAZ's system once magic activates the failsafe activates the backup anti-magic generators! But this risks activating all failsafes. In that case the secondary (and any tertiary) backups could be placed on a timed delay. The delay can be reset after any level 1 failure.

A word to the wise: it's always good to regularly test and/or (safely) simulate failures to make sure your system continues working as a whole. And don't buy the whole set of anti-magic generators from the same supplier, use different suppliers and from different batches so you don't have the entire lot failing at once!

Answer 6

Deal with fire by fire

Here is a simple, yet effective way to deal with them all. just lace the whole iceberg (or just every single spell) with scroll that would in the precense of any magic ignite & explode. like that you are insured that all of those prionier die in the explosion or get in the freezing cold water (which is a death sentence in the matter of a few minute.)

if you want something a bit less flashy, but who would save the life of gards, a simple colar that would explode /ignite in a similar fashion would probably do the trick too, but there is some slight chance that a priosnier or two tamper with there own

Location is everything

alternatively, if you could place your prison in an area where the absence of magic is actually helping the people to survive... the dynamic would be very different. either have the magic in this part of the world being too wild, tainted or untamed. causing mage expaused to it to go mad or just die/kill themself while your gard are scot free. If you allow the latter to die, this region being prown to the appearance of daemon can be a neet way too insure nobody want to shut down the defences.