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The players in my Curse of Strahd game have reached the Amber Temple. While there, they recently breached a certain door, for which the module indicates

If the doors are reduced to 0 hit points, a greater invisibility spell is cast on the amber golem in this room. The spell lasts for 1 minute.

One of the PC's is a wizard, and he had a counterspell prepared and a slot available. Initially I thought he might be able to counter this effect, but reading the description of counterspell it states (emphasis mine)

You attempt to interrupt a creature in the process of casting a spell. If the creature is casting a spell of 3rd level or lower, its spell fails and has no effect. If it is casting a spell of 4th level or higher, make an ability check using your spellcasting ability. The DC equals 10 + the spell's level. On a success, the creature's spell fails and has no effect.

Clearly counterspell targets only spells cast by creatures, and this effect doesn't appear to have a caster. The use of the passive voice, "a...spell is cast" seems to imply that there is not a caster, and the statement that the spell will last for one minute perhaps is underscoring that point, since if the spell had a caster, that caster would need to maintain Concentration.

It does not even appear to be the case that a specific magic item is casting the spell or granting a spell-like effect, for which there is some guidance in the DMG (and see this question on spells cast by magic items).

Thus as a DM I feel at a loss for how to adjudicate interactions with this spell, both with counterspell and anything else the players might attempt.

After the session I tried to explore whether there was guidance or precedent for spells without casters. I checked the DMG section on creating spells (starts on 283) but didn't find anything.

Drawing on my experience from earlier editions, I figured if something produced a spell like that it would likely be from a contingency, limited wish, or wish. Checking the 5e descriptions, contingency makes it clear that it can only trigger spells cast on oneself, and furthermore has a duration of just 10 days. It appears that limited wish does not exist in 5e. And wish, the "mightiest spell a mortal creature can cast", does not explicitly permit any effects with a duration beyond instantaneous except for the duplication of existing spells.

Is there any guidance or precedent for adjudicating a spell without a caster?
Is there any indication of how such an effect is produced?

Or is this just part of "The GM describes the environment" and I should rule by context-informed fiat?

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  • \$\begingroup\$ @MarkWells Per the optional rules in XGtE, a spell can be noted by its components when cast. Are there components when a spell "is cast" without a caster? Trying to determine whether there would be something to notice is one of the purposes of this question. Absent an understanding of casterless spells, I ruled at the time that the wizard was allowed an Arcana role to sense a disturbance in the weave. \$\endgroup\$ – Kirt Dec 27 '20 at 21:50
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The best example of this I could think of is Glyph of Warding, which can automatically cast a harmful spell on the creature triggering it.

This has obvious similarities to the situation you describe; most importantly, no creature is visible casting the spell when it is cast.

In response to this October 2016 tweet by a user asking whether a Glyph of Warding can be Counterspelled when it casts its stored spell, rules designer Jeremy Crawford (unofficially) ruled that it can not:

The casting time of counterspell specifies that you're responding to a creature you can see who is casting a spell.

Counterspell needs a creature to be casting the spell that is to be cancelled. The tweet makes that clear. If there is no creature casting the spell, there is no possibility to Counterspell it.

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    \$\begingroup\$ While this doesn't address the larger issue of casterless spells, I agree that comparing the effect to a glyph is useful for adjudicating interactions. I think this answer could be improved with a side-by-side comparison of what is similar and what is different between a glyph and the effects described in the module. \$\endgroup\$ – Kirt Dec 27 '20 at 21:47
  • \$\begingroup\$ @Kirt, It doesn't? If there is no caster casting the spell then counterspell isn't possible. That seems to be a general rule affecting all cases. \$\endgroup\$ – Steve Dec 27 '20 at 22:43
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    \$\begingroup\$ Tweets aren't "Sage Advice". That's just the name of a third-party site that compiles designer tweets. I've edited the answer to link directly to the mentioned tweets and clarify the description. \$\endgroup\$ – V2Blast Dec 28 '20 at 0:10
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    \$\begingroup\$ @Kirt There's actually nothing available to players that interacts with spells being cast, other than counterspell. If, hypothetically, there was, then, hypothetically, it would have its own set of gameplay issues that would need to be considered. There's no reason to try to come up with some general framework for "casterless spells". \$\endgroup\$ – Mark Wells Dec 28 '20 at 0:59
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    \$\begingroup\$ @MarkWells I can think of at least "Identifying a spell being cast" from XGtE. But to your point, if this is not something that the rules address, then that is part of the answer to my question; "The rules do not specifically address casterless spells." Simply stating that would answer part of my question and improve any answer that contained it. \$\endgroup\$ – Kirt Dec 28 '20 at 1:35
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In this case, you have no way to know a spell is being cast.

Usually you know someone's casting a spell because you can hear them saying the words or see them performing mystic signs or something. In this case there's none of that. The room just casts the spell, like a kind of autonomous magic item. So the characters can't cast counterspell because they don't know that there's a spell.

(This fits with the function of the trap, which is to make the room appear unguarded, so that the intruders will come inside and get torn apart by the golem.)

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  • \$\begingroup\$ Agreed that no components means the PC's would not notice the spell being cast. However, nothing in the text suggests that it is the room, specifically, that is casting the spell. \$\endgroup\$ – Kirt Dec 27 '20 at 21:58
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    \$\begingroup\$ It doesn't need to be "the room, specifically". The room, the door, some device under the floor, whatever. The point is that it's not a conventional spellcaster who needs to say the words. Think of how you narrated it at the table: what about this situation indicates that a spell is being cast? \$\endgroup\$ – Mark Wells Dec 27 '20 at 22:41

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