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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 '18 at 8:52
  • \$\begingroup\$ @SeriousBri No. Reading the whole spell description, it’s clear that the target must do the triggering, not a third party. \$\endgroup\$ – SevenSidedDie Jul 4 '18 at 14:46
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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 '18 at 10:26

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