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The 5th-level Spell geas allows the caster to Charm a creature, and also give direct commands to it. Unlike some Charm spells, geas does not have a restriction that prevents a creature who passed their saving throw from having to make another saving throw if the spell is cast again upon them. So theoretically, a setup that casts the spell over and over on the same creature would, pending legendary resistances or straight-up immunity, nearly guarantee that one of the spells triggers successfully. But if the spell already cast successfully, you wouldn't want to keep casting it; once the creature is properly charmed, you'd want to stop casting the spell.

So consider the following setup: A Wizard uses glyph of warding to inscribe geas into the wall of their lab, sets the trigger to "When I speak a command phrase and point at a creature, target the creature I pointed at". Then, they inscribe a second glyph of warding with geas, and its trigger is "if glyph of warding A casts a spell, and the creature targeted passes its saving throw, then target that creature". And then they inscribe a third "if glyph of warding B casts a spell, and..." and so on, and so on.

My question is, is this valid RAW? Although the usual principle is that "Spells only do what they say they do", the description for glyph of warding is deliberately written to invoke some degree of ambiguity. To wit:

You decide what triggers the glyph when you cast the spell. For glyphs inscribed on a surface, the most typical triggers include touching or standing on the glyph, removing another object covering the glyph, approaching within a certain distance of the glyph, or manipulating the object on which the glyph is inscribed. For glyphs inscribed within an object, the most common triggers include opening that object, approaching within a certain distance of the object, or seeing or reading the glyph. Once a glyph is triggered, this spell ends.

You can further refine the trigger so the spell activates only under certain circumstances or according to physical characteristics (such as height or weight), creature kind (for example, the ward could be set to affect aberrations or drow), or alignment. You can also set conditions for creatures that don't trigger the glyph, such as those who say a certain password.

—PHB, pg.245

It's easy to see how the first criterion is allowed, but the second criterion depends on whether a glyph understands what a saving throw is, which itself is an abstraction of a creature's ability to dodge/resist/ignore the effects of a spell. One could rephrase the command as "if the creature is not affected by the spell, trigger on that creature", but that still depends on some dodgy ruling, i.e. can the glyph detect that a creature was not affected by the spell?

Is there a more general rule that describes the upper limits on what the glyph of warding can detect as valid targets, or how complex its triggers are allowed to be?

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This will not work for two reasons.

1. That trigger makes you the target

By making the trigger be something you do, you become the target. You can’t give the glyph instructions otherwise.

If the spell has a target, it targets the creature that triggered the glyph.

2. The glyph has no capacity to detect specific magical effects or saving throw states on creatures

To make a creature trigger the glyph, it must satisfy the conditions of the trigger. The basic trigger is an act, and the optional refining condition is something about the creature. However, neither a glyph triggering is a suitable trigger action (that would make the other glyph the target, but it isn’t a creature anyway), nor is suffering a spell effect an act.

Furthermore, even if an act could somehow be chosen to coordinate the glyph chain cascade (perhaps proximity, with fractions of inches closer for subsequent glyphs), you could not halt the cascade by checking if the creature is already under a geas, because that’s not a physical characteristic of the creature, a creature kind, nor an alignment:

You can further refine the trigger so the spell activates only under certain circumstances or according to physical characteristics (such as height or weight), creature kind (for example, the ward could be set to affect aberrations or drow), or alignment.

You’ll just have to settle for autonomous, non-guided glyph barrages that don’t know when to stop.

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  • \$\begingroup\$ So the second target can't trigger the glyph 'by being pointed at'? I would rule being pointed as is perceivable, albeit external to the target. \$\endgroup\$
    – SeriousBri
    Jul 4, 2018 at 8:52
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    \$\begingroup\$ @SeriousBri No. Reading the whole spell description, it’s clear that the target must do the triggering, not a third party. \$\endgroup\$ Jul 4, 2018 at 14:46
  • \$\begingroup\$ Being under effects of a spell does comprise a "circumstance" though; which is an allowed condition of triggering, listed distinctly separate from "physical characteristics", "creature kind", & "alignment". The trigger could be specified as "When a creature I am pointing is within range," (creature is the target), & the circumstance could be "which is not under a Geas" (note that this could be ANYONE'S Geas). This targets a creature pointed at which is not under a Geas; & these conditions are at least as detectable as a creature "seeing" it. Spells can function or fail based on status effects. \$\endgroup\$ Jul 24 at 16:50
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Saves are not identifiable

Unless there is a perceptible effect from a spell the target might not even know they have resisted, so the Glyph won't be able to tell.

Other spells, such as charm person, display no visible, audible, or otherwise perceptible sign of their effects, and could easily go unnoticed by someone unaffected by them. As noted in the Player’s Handbook, you normally don’t know that a spell has been cast unless the spell produces a noticeable effect. (Perceiving a caster at work. XGTE p85)

There is a way around this, even if it does not work

Glyph 1 goes off and as part of the Geas you get them to immediately say a phrase / word.

Glyph 2 has a trigger of "same trigger as glyph 1 and phrase x not said within 1 second". The Geas from Glyph 2 also gets the target to say a different phrase / word.

Glyph 3 has a trigger of "same trigger as glyph 1 and phrase x or y not said within 2 seconds". The Geas from Glyph 3 also gets the target to say a different phrase / word.

Repeat...

In this case Glyph 1 will go off, if successful the target will say the phrase as part of the Geas, and not qualify as a trigger for the other Glyph's. If the Geas fails then they won't say the phrase and thus will quality for the trigger on Glyph 2 and Glyph 3. Glyph 2 will immediately go off, while Glyph 3 will be waiting to see if the phrase from Geas 2 is spoken, going off if necessary.

You can have as many as you need, and only need to reset those that have gone off.

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  • \$\begingroup\$ A fine solution. This set of triggers misses the potential for the creature to choose to take the 5d10 psychic damage (which only happens once per day), but whether or not that damage is perceivable is debatable. \$\endgroup\$
    – illustro
    Jul 5, 2018 at 10:26
  • \$\begingroup\$ There is no mention that the caster or target need to be aware of a circumstance, in order for the spell to function based on that circumstance: Casting Resurrection does not require you to know that a targeted creature is dead; it merely functions only on dead creatures. Dispel Magic can end effects, without any creature knowing those effects. Spells do not rely on natural senses unless they say they do. Spells can (& do) succeed or fail (or trigger!) based upon outwardly non-detected criteria. Indeed, Glyph Of Warding explicitly allows conditionals like "sees" & "alignment" & "circumstance". \$\endgroup\$ Jul 24 at 17:10
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    \$\begingroup\$ 'Saves are not identifiable' to PC's, which is what the context is for the quote from XGtE. That quote does not address whether or not spells can detect saves. However, the geas forcing the target to perform the trigger for the next glyph is a nice solution. It does beg the question of whether they know the nature of the geas on a successful save, though, and could thus 'fake' compliance to preempt a subsequent casting, \$\endgroup\$
    – Kirt
    Jul 24 at 23:17
  • \$\begingroup\$ Agreed that having Geas' caster-specified conditionals prevent the triggering of the reserve Glyphs (via safewords, in particular) could be a nice workaround, in the event one's GM fails to acknowledge the lack of sensory requirements for spells to have their effects. \$\endgroup\$ Jul 25 at 5:01
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The wording of the Glyph Of Warding spell, places no clear limitation regarding what conditionals can be placed on its triggering.

The wording implies that the conditionals can be almost anything. Proscriptive interpretation still yields no firm limits:

Unless specified in their listing, 5e spells do not rely on natural senses for the fulfillment of their descriptions. As an example, a spell that affects only Fiends or Fey does not require the caster (or the target) to know whether a given target is Fiend or Fey. Spells which affect all creatures that see something, do not require any means for the spell (or spellcaster) to know whether that thing has been seen. Spells affecting only creatures below a certain HP, or which are under a certain status effect, abound; yet these spells do not require the caster (or target) to know these values. The spell's description determines what values are relevant.

It is entirely possible to cast a spell which succeeds or fails based upon criteria the caster has no (other) means of detecting. Again, spells only rely upon senses to the extent specified in their descriptions.

The conditionals given as examples within Glyph Of Warding's description, include things the caster has no natural means of knowing:

  1. Whether a creature "sees" the glyph, is a primary trigger type.

  2. A creature's "alignment", is listed as one form of trigger conditional.

Unless the intent is that the caster must be present for the Glyph to function, the Glyph spell must necessarily determine whether these criteria are true. 5e spells are not sentient by default, nor do they have "senses" defined except as specified within the context of a spell's effects. In short, ordinary spells are not creatures in 5e, & do not rely upon natural senses to detect the conditionals of their descriptions. For example, Animal Friendship does not use "senses" to detect whether a targeted creature has more than 4 Intelligence (nor does it inform the caster).

Effectively: 5e spells function independently of "senses" unless specified, & a great many spells function differently based upon criteria which are not ordinarily subject to "senses".

So, if a spell's description specifies that only undead of evil alignment with less than 5 HP, will take damage, that conditional is flawlessly applied regardless of whether the spell has any obvious means of detecting that conditional's value.

Spells which allow the caster to specify conditionals, state the limitations of those conditionals in their description:

For example, the spell Geas, specifies that a creature cannot be commanded to kill itself, & the spell Command, details the effects of precisely five words devoid of GM fiat.

In contrast, Glyph Of Warding gives examples which include conditionals the caster may have no other means of determining.

Glyph Of Warding could be used as a test of alignment, which is otherwise not subject to detection by natural "senses". Glyph Of Warding can be triggered by a creature that is Immaterial and Invisible and fully Hidden and under the effects of Nondetection.

Contrary to other spell descriptions which give examples & limitations, Glyph Of Warding gives diverse examples without specifying limitations, regarding its possible triggers.

So far as I've found, there is no specific or general rule which limits the conditionals allowable via Glyph Of Warding, nor do we have any errata or rulings by Crawford et al, to guide us.

Using other spells as examples, we can see that if such limitations existed, they should be specified within the spell description.

Therefore, the conditionals which can be specified as triggers for a Glyph Of Warding, are limited only by GM fiat.
Canonically, no limitation on the triggers has been specified yet.

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    \$\begingroup\$ That spells don't work on some creatures by definition, and that Glyph can detect things like alignment, seem to me to be two separate issues. A spell doesn't necessarily 'know' on what creatures it will function - it could just work or not. But Glyph absolutely does have to be able to detect information about its target, and that is what the question is about. I think this could be a good answer following a major re-write that emphasized examples of what a Glyph can actually detect. \$\endgroup\$
    – Kirt
    Jul 24 at 23:03
  • \$\begingroup\$ We already have those examples; & Glyph only needs to "know" things for the same reasons any spell needs to know those things; shall I quote comparable spells verbatim? \$\endgroup\$ Jul 25 at 4:37
  • \$\begingroup\$ (Essentially, spells only do what they say they do, & Glyph doesn't say it needs eyes to see or scrying to determine alignment. It is a spell & spell mechanics work as specified, without need of character knowledge methods. No limitations are specified, therefore I'm unsure what argument should be further emphasized?) \$\endgroup\$ Jul 25 at 4:46
  • \$\begingroup\$ Apologies for the inefficient comments. I will revisit my attempt say an Answer here, when I am at a desktop browser interface. \$\endgroup\$ Jul 25 at 4:50
  • \$\begingroup\$ Example of what Glyph can detect: circumstance \$\endgroup\$ Jul 25 at 5:06

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