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?
Best Answer
There is no limit to the trigger, but the spell may fail
Please stop being evil's answer shows well that there is no actual distance limit to the triggering event. This answer argues that there is still a practical distance limit for most purposes.
I assume in this answer that the spell is being cast from the glyph's space for the purpose of targeting.
The text is fairly clear that the effect of explosive runes is centered on the glyph. A spell glyph is a little less straightforward, but there are still limitations.
Lets use this in the context of your Drow alarm example. We know that the spell is cast immediately when the triggering condition is met, and since the spell doesn't say otherwise, one mile is a viable distance. However, the area of effect must be centered on the creature, and there is no point of origin that can accomplish this within the spell's range. Thus, even though the glyph is triggered, when the spell is cast, the necessary target is not in range. When there is no valid target, the spell usually fails and is wasted, see this answer for details.
Your plan is clever, and though I would allow it at my table, there is no RAW reason for the glyph to 'aim' as close as possible, so by the rules, the glyph would be spent, with no boom. Note that an explosive rune does not target anything, so an explosive rune using thunder magic could reasonably still provide an audible alarm, due to the sound based nature of thunder damage. By monitoring a summons glyph, you could identify the direction of the Drow by where the summons appear, since they are specifically stated to appear "as close as possible" to the triggering creature.
By contrast, in your Fireball example, the triggering creature is within the spell's range. The area 'chosen' by the glyph (centered on triggering creature) and the area possible from the Fireball spell (point of origin within 150 ft of the spell's casting) do not contradict each other, so the spell succeeds.