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.