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The conventional usage of a glyph of warding spell, exemplified by the "explosive runes" variant, is something like a magical land mine: step on or near it and it explodes, dealing damage in a small radius around the glyph itself. However, if one stores a spell with a long range and uses an appropriate trigger condition, it seems that the spell glyph form of glyph of warding could be used more like a single-shot turret. Consider these examples:

  • One could create a spell glyph containing a fireball spell (range 150 ft) with a trigger of "when a small or larger creature approaches within 150 feet" and place the glyph in the center of a room 300 feet across to effectively ward the entire room with a single glyph.

  • Going even further, one could imagine using a spell glyph as a magical alarm system, storing a loud spell (e.g. thunderwave or shatter) with a trigger of "when a drow approaches within 1 mile". Such a glyph could act as an early warning system covering an entire castle or dungeon. (For bonus points, exploit the fact that the stored spell will be aimed in the direction of the triggering creature by placing bells of different pitches in the cardinal directions around the glyph, then determine the direction of approach based on which bell(s) ring the loudest.)

Either of both of these examples seems like they could be outside the intended power level for this 3rd level spell, but as far as I can tell, the spell's text doesn't explicitly rule out either example. In fact, I've intentionally phrased the above triggers to conform to the specific examples of "typical triggers" in the description (highlighted in bold):

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. [...]

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.

The lack of any apparent range limit is especially notable given that some spells with similar triggering mechanics do given an explicit range limit. For example, magic mouth requires the triggering circumstance to be observable within 30 feet:

The triggering circumstance can be as general or as detailed as you like, though it must be based on visual or audible conditions that occur within 30 feet of the object.

So, what, if anything, limits the "detection range" of the trigger for glyph of warding? Do either of the above examples work, or is there some part of the spell's text, or some applicable rule, that prevents the glyph from detecting the triggering condition at that range? Is this entirely up to the DM's discretion?

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There is no detection range

Some triggers may not be allowable, but if a trigger is allowed, it functions at any range. That said, triggers probably must have to do with interacting with the warded object or surface in some way (c.f. Can your own death be a trigger for Glyph of Warding?). Most of Glyph of Warding's power comes from it's being able to refine other spells to exceed normal use restrictions, so it's not too surprising that its refinement system is a lot more open-ended than e.g. magic mouth.

Not also that the idea inherent in the question that spell effects take place with their ordinary range restrictions is not ironclad. I assume you got that idea from here, but the answer to that question is merely one of many possible RAW-consistent answers and in practice different groups will have radically different interpretations of range limitations for this spell. The mechanics, as you have presumably noticed by now, are a bit under-specified, so there's a lot of work for DMs to fill in that's relevant to whether or not your long-range detection systems work.

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  • \$\begingroup\$ In my examples, I've assumed that the when the stored spell is released, it is limited by its normal range, but that's really tangential to the question at hand. The spell glyph only becomes even more powerful if you remove that assumption. In any case, do you think the specific examples given would be allowable, or are you saying that it's up to the DM to decide whether to allow such long-range triggers? \$\endgroup\$ – Ryan Thompson Feb 21 at 5:06

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