The fifth edition version of zone of truth can cause many problems in mystery-oriented scenarios. This second level enchantment available to bards, clerics, and paladins takes an action to cast and lasts for ten minutes.

The issues with the spell - when used by a clever investigator - are numerous. These reddit posts elaborate on the spell in longer form (the comments contain excellent discussions as well), but to summarize

1. Guilty persons under direct questioning have many forms of evasion, but rely on politics & law to protect them from being forced to be straightforward

2. It’s an absolute alibi - if you have a complete list of possible suspects, zone of truth allows you to procedurally eliminate them

3. Combining the previous two points, legal systems with ready access to second level enchantment spells might readily engage in brutal inquisitions, relying on zone of truth to “protect the innocent” — if you can’t freely disclaim your innocence under the effects of the spell, you must be guilty

Defenses against the spell are limited; as an enchantment spell, nondetection (the usual criminal mastermind’s solution) won’t help, nor seemingly would the far more powerful mind blank. The two chief ways to fool a zone of truth are a Ring of Mind Shielding, a major uncommon item, and a Rakshasa’s Limited Magic Immunity. A few other magic items, such as the knave’s eye patch and lord’s ensemble from Waterdeep: Dragon Heist, provide similar functionality at higher rarities.

Some (such as the previously linked reddit posts) seem to believe that immunity to a zone of truth means you can’t make the Charisma saving throw at all, and thus is automatically detected by the spellcaster. I differ in this interpretation — similar to how fire immune creatures still make saving throws against a fireball, creatures immune to a zone of truth can still make and fail the saving throw, then lie freely as they are immune to the effect, not to needing to make a saving throw. This fits with RAI, as an effect that explicitly protects against zone of truth wouldn’t be much help if it didn’t let you fully trick the spellcaster. The overly elaborate wording of the Mastermind Rogue’s Soul of Deceit feature is just part of Wizard’s inconsistent language and individual writer’s tendency to overelaborate what a feature does if they have room on the page to expound.

Even without these special defenses, the spell isn’t foolproof — one must still be capable of subjecting all possible suspects to the zone of truth, and many mystery plot twists leverage the fact that each participant is telling the truth as they understand it. To illustrate this point, let’s look at how zone of truth would alter Benoit Blanc’s investigations (Spoilers for Knives Out and Glass Onion!).

Knives Out. Our point of view character, Marta, is in some sense under a zone of truth spell for the duration of the movie with her signature vomiting reaction. Furthermore, from the outset it seems like the family patriarch Harlan Thrombey died by suicide. The front half of the movie would likely proceed in a fairly similar fashion even with a zone of truth, as the lies and cover-ups told by the family members are immaterial to the murder — Joni’s double dealing and Richard’s infidelity being the chief ones — and are immediately seen through by Benoit. However, these are an excellent example of why those with power and money might prefer a less precise justice system because even exculpatory testimony can be otherwise scandalous. Ransom’s late arrival also showcases an important caveat to zone of truth, as someone must be present to provide testimony under its effects. Without Benoit’s dogged pursuit of the whole truth, Knives Out with zone of truth would likely end with Marta losing the fortune accorded to her by Harlan’s will, her mother’s deportation, and her at a minimum losing her licensing as a nurse let alone possible jail time for medical malpractice. Fortunately, Benoit sees through Ransom’s schemes and helps Marta pin him as the killer; while zone of truth may have expedited some of this, especially if Ransom could be forced to provide testimony, the twists and turns of the plot innately provide a degree of resilience to this magic.

Glass Onion. While Rian Johnson’s first Benoit Blanc film would be resilient, Glass Onion would shatter in its presence. To start, the entire film happens due to five people committing blatant perjury to steal Alpha away from Andi — something zone of truth would handily solve in such a high-stakes legal proceeding. Even on the island, the mystery is very straightforward once you cross the threshold of being able to use the spell (which would admittedly be tricky - until Duke’s murder, the fact that there is an investigation at all is a secret; after his murder, compelling the suspects to not be chaotic idiots was seemingly impossible, although Lionel and Claire would probably quickly comply to clear their names).

TL;DR. Zone of truth is a powerful spell, and while it is certainly possible to write a mystery that still functions in the face of it, plenty of mysteries rely on people blatantly lying in testimonial circumstances. At the low cost of a second level spell slot per ten minutes of testimony, a party of adventurers can cause serious problems for an unprepared DM.

How Did We Get Here

In third edition, the zone of truth spell was far more limited. It was still a second level spell that took an action to cast, and actually had a radius of 20’, but the duration was limited to 1 minute per level of the spellcaster and had the following rules text:

There are two key differences from the 5th edition version of the spell

• A creature only makes the saving throw once - standing in the zone does not force repeats every 6 seconds

• The spellcaster does not innately know whether the subject passed their saving throw

Trying to be wily in the face of zone of truth was far more likely to work in third edition. If you pass the initial saving throw, you can simply pretend to be under the spell’s effects. The interrogator is not without all recourse, as a DC 25 Sense Motive check over the course of a minute can confirm the subject is under an enchantment, as can various forms of divination magic. However, the bar for rural investigators is raised significantly, as a DC 25 check is not an easy bar to clear for low level characters (Bonuses to skill checks were much easier to acquire in 3rd edition, but would still take until level 6+ to hope to reliably hit 25+). Furthermore, Detect Magic takes three rounds per target to “ramp up” and confirm the presence of the enchantment spell, which could furthermore be masked by the presence of other enchantment magic.

In the Eberron campaign setting, zone of truth was worked into the basic methods of law and order. Sharn: City of Towers provided the rules for a magic item known as the eye of Aureon.

In the city of Sharn, magistrates use an eye of Aureon to quickly process cases. Other nations or cultures might institute different processes, but this forms a reasonable template for what a society with ready access to zone of truth might be capable of.

The following section then describes trial procedures, including various magical precautions such as the removal of possibly-enchanted jewelry. Like the real world, criminal trials would be rare, as the initial zone of truth is relied on to sort through the likely-guilty versus not-guilty; however, the weaknesses in the 3.5 version of zone of truth make it possible to pass the DC 20 Will saving throw, disclaim your innocence to the magistrate in the first fourteen minutes, and then hopefully have your case dismissed.

Tangentially, Eberron offers several alternatives to this style of justice system, namely Zilargo and Riedra; both operate intrusive and extensive domestic surveillance programs, not even requiring zone of truth to sort through events afterwards.

4th Edition. The 2008 Player’s Handbook doesn’t have a spell named zone of truth; the closest equivalent is the “Discern Lies” ritual, which let the spellcaster massively improve their “Insight checks to discern any untruths spoken in your presence during the duration”. This ritual has a ten minute casting time, only lasts for five minutes, and costs 140 gold in material components per cast; in short, while certainly useful, it doesn’t have the wide-reaching effects that either the third or fifth edition zone of truth spells can have.

Why Have Zone of Truth At All. D&D ostensibly supports three pillars of play - combat, social, and exploration. On a surface level, zone of truth is exclusively targeted at supporting the social layer of the game - it’s a spell that’s not meant for combat or exploration. Digging a bit deeper, zone of truth seems to undermine the social pillar, both in a mechanical sense by skipping over the insight skill as well as from a narrative standpoint by its ability to blow up many forms of mystery plots.

At the bedrock level, however, zone of truth supports what D&D is best at - heroic fantasy. It’s not a dedicated mystery system, it’s where a mystery is an adventure amongst many. D&D was not written to be a legal procedural; zone of truth allows parties to put a bow on the resolution of a mystery, confident that they’ve nabbed the right guy so they can move on with their heroics. From that perspective, the overwhelming strength of zone of truth is intentional — the party is the only one around who’s making things better, and this spell helps them clean things up. Furthermore, as a non-ritual spell it can eat into party resources on a busy day, on top of needing to be prepared by a cleric or paladin, let alone taking an entire known spell of a bard.

Dealing with Zone of Truth

For better or for worse, D&D is not just a game about fantasy heroics, and the zone of truth spell as written may prove more of an impediment to a good gaming experience than a boon. Here are some ideas for how to make zone of truth more interesting.

Legal Protection. Forcing someone to speak in a zone of truth requires the party to have some form of power over the person; that might be in the form of immediate threats (we’ll kill you if you don’t cooperate) or more deliberation (we’ll do this in front of friendly law enforcement who have the power of the state behind them). If that isn’t true — for example a well-resourced villain who can bring down a harsh reprisal on the party if they’re harmed — the subject can simply not speak. As illustrated by the Knives Out example, even people who are not guilty can have other things to hide and thus refuse to testify in the zone of truth. Alternatively, someone might simply prefer to face the consequences of not testifying themselves rather than spill the beans. This could be because they’re particularly fanatical, or because they’re afraid of a reprisal by their soon-to-be-former-allies against themselves or people they care about.

Magical Protection. The well-to-do and criminally prepared are likely to have a ring of mind shielding, which has a convenient function of being able to turn invisible. Furthermore, repeated castings of arcanist’s magic aura can make a ring appear nonmagical, fooling a party relying on detect magic or see invisibility. Unfortunately, these protections are less likely to help in more formal settings, such as a trial in Sharn, where precautions like pat downs to remove jewelry make it harder to conceal a ring, nor do they protect those who for whatever reason don’t have a ring of mind shielding.

Depending on the nature of a campaign setting and what needs to be concealed, consensual use of the modify memory spell might also allow a creature to pass a zone of truth investigation. Unfortunately, the 5th level spell slot minimum and timing restrictions limit its effectiveness at concealing any form of complex lie.

Going beyond RAW, changing the nondetection and/or mind blank spells to protect against zone of truth would allow spellcasters to reliably provide an answer. Nondetection especially would undermine the reliability of zone of truth, as the spell itself protects from detect magic like effects that would discern if the speaker is otherwise protected. A much more limited approach might be to offer a cheaper version of the ring of mind shielding that only provides a fraction of the ring’s features, such as not including the soul-storing functionality.

Corruption. A critical lynchpin of the 5e zone of truth setup is the person validating the quality of a zone of truth is the spellcaster of the zone. RAW, neither the subject nor caster can decide on its efficacy; a creature that miraculously passes all of its saving throws can never be validated as telling the truth, nor can the spellcaster allow a creature to automatically succeed. To allow for corrupt spellcasters, there might be a lesser known version of the spell that simply allows the spellcaster to designate creatures within the zone as exempt from its effect; a successful Intelligence (Arcana) check could distinguish between the two versions, as per the XGTE spell identification rules.

A second form of corruption could lean on zone of truth being an enchantment spell. Rather than (or perhaps in addition to) enabling perjury, a corrupt spellcaster could add additional enchantment magic to compel someone to provide false testimony. This could range from charm style effects that guide someone to testify in a favorable fashion to outright memory modification. This might be performed with existing enchantment spells such as charm person, suggestion, or modify memory, or it might be a variant zone of truth that incorporates those effects into itself.

Changing the Spell. The Level Up Adventurer’s Guide by EN publishing made substantial changes to the zone of truth spell, offering the following text as an alternative to WotC’s version:

For a DM who simply wants to change Zone of Truth, rather than their antagonists, this is by far the simplest solution; the DC 10-16 skill check is difficult to maintain with repeated lies, but gives a subject a real chance on individually misleading statements. To combine it with the idea of a corrupt spellcaster, one could have a variant version in the style of a5e’s “rare spells” that allows the spellcaster to publicly malign an affected creature as having lied when they did not:

Jurian’s Corrupted Zone of Truth. The spellcaster can manually activate the psychic damage once per minute, giving the impression that a true statement was instead false.

What if… ZoT was never perfect. For a campaign drenched in shades of gray, the common use of zone of truth by law enforcement can still be a fact of life. In the real world, law enforcement officials have frequently oversold the reliability of their techniques, especially in media (part of what scholars call Copaganda). Revealing flaws in zone of truth could be an entire campaign arc, from natural issues with its reliability to the deliberate use of zone of truth to mask blatant perjury by law enforcement. Here are some quick ideas for how zone of truth might be flawed.

• The spell is unreliable at certain times, such as a particular phase of the moon

• Some types of objects or materials interfere with the spell

• Everyone knows that zone of truth still allows defendants to be misleading, but the city guard use it to give false sincerity to their statements.

To elaborate on the first two ideas, a flaw in the spell might make it overly permissive (during the full moon you can freely lie about when things happened) or restrictive (if you have soot in your clothing you can’t state your intentions, only what actually happened). These rule alterations should support your worldbuilding and themes of your plot, such as highlighting class differences, and be discoverable by your party - these should not be arbitrary alterations and otherwise disconnected from your storytelling.

Conclusion

Most DMs who run mystery scenarios will run into the harsh reality that zone of truth risks prematurely ending their plot. Fortunately, there are tools both within the rules-as-written as well as through clever modifications that can allow you to turn an adventure-ender into something more dynamic and interesting.