DND 5E Crown of Madness – How You Can Trigger It Using Glyph of Warding

dnd-5espellstraps

Let's say that:

I cast Glyph of Warding and imbue Crown of Madness into it, with the
trigger being a creature in a party of 2 or greater steps on the
glyph.
A party of 2 walks over the glyph and the first creature triggers
the glyph.
The creature fails his Wisdom saving throw.
I, the caster, am nowhere near the glyph, meaning I have no sight of
the creatures and no way to know the glyph has been triggered.

A. Can I still mentally choose a target for the creature to attack?

B. Can I make him attack his buddy?

Best Answer

One way or another, crown of madness is not going to work. Either:

When the spell says "you", it means the glyph of warding, in which case crown of madness will work, but you won't be able to choose a target, so the victim will be able to act normally.
When the spell says "you", it means you, the caster, in which case, you can't see the target, so crown of madness won't work at all.
When the spell says "you", it means the glyph of warding, and glyph of warding is capable of mentally choosing targets, in which case crown of madness will work, but glyph of warding will always target the creature who triggered the glyph, and crown of madness doesn't allow you to command a creature to attack itself.

Yes, this would work

The way you have the trigger worded is perfectly valid and will result in the creature getting hit with all 100 fireball spells in/on the book.

I would tweak the pages' trigger to say "when a creature causes these pages to take fire damage" just to be extra clear that the creature that triggers the cover also triggers the pages.

However...you can't move the book more than 10 feet

Glyph of Warding says:

If the surface or object is moved more than 10 feet from where you cast the spell, the glyph is broken, and the spell ends without being triggered.

So, with this trick, not only will you have to spend 100 hours and 20,000 gp on the spells needed, you will also not be able to move more than 10 feet the entire time. And then you have to hope that your enemy comes to you.

Certainly with enough planning, preparation, and a lot of luck this could be a strategic play. However, there is almost no chance that this strategy will work for the average adventurer.

Will using a portable hole help avoid this? Up to the DM.

Object/surface in a portable hole (or other extradimensional space)

It has been proposed that you might be able to get around this by using a portable hole or some other extraplanar storage see (this Q&A). This might work, except a portable hole is 10 feet deep so removing an item from it would still trigger it.

Also, a DM could argue that the extradimensional space of the hole is an infinite distance away from the current plane. Thus, taking it out or putting it in the hole would break the glyph regardless. So this comes down to a DM decision to make that call.

Triggering object/surface while still in a portable hole (or other extradimensional space)

Another way would be to mount it on a surface inside a portable hole. It is very open-ended what is considered a surface for this spell (see this Q&A) but mounting it to the side of the extradimensional space is probably perfectly acceptable. Of course, since you can't move the surface/object out of the space (see previous section) you are not limited to triggering while still inside the hole.

And this should work as expected (assuming you set an appropriate trigger), doing damage so everyone and everything in the blast radius of the fireballs.

However, I would be very cautious since the portable hole is an object made out of cloth per the description:

This fine black cloth, soft as silk, is folded up to the dimensions of a handkerchief.

Thus, it would potentially not be terribly hard to destroy with 100 fireballs worth of damage being triggered inside of/around it (since fireball's radius well exceeds the portable hole's dimensions).

Bottom line: it will be up to the DM if and how well any workarounds work

There are probably other ways around it, but you'll need a DM to likely agree to whatever workaround you come up with. And if they do watch out because...

Sounds like a great tactic for a villain!¹

While this strategy is likely never to work for an adventurer, it is perfect for a villain setting a trap. If a DM is happy to let this work for you as a PC, they may be even happier to allow it to work for the evil NPCs as well.

^{1 - Thanks @Erik}

D&D 5E – Damage from Simultaneous Fireball Hits

They take damage from both fireballs

The key for me is the word "effects" from your quote:

The effects of the same spell cast multiple times...

I would argue that the damage dealt from the fireball is not an "effect" in the context of what your quote is talking about. This spell simply deals damage and has no additional "effects".

Furthermore (as pointed out by @PJRZ), the section you quote from begins with this sentence:

The effects of different spells add together while the durations of those spells overlap. (PHB, pg. 205)

This suggests that spells that simply have an instantaneous effect do not come under this rule, this they effectively have no durations. This also confirms that the context I mention above is for ongoing effects of a spell, where considering overlapping bonuses or additional effects makes more sense.

Instantaneous isn't a real duration, it's just a category of duration for the purposes of that property of a spell

Instantaneous means it has no duration; as for the duration of a spell being listed as instantaneous, I understand duration in this context to simply mean the name of a category, a property of the spell, but it isn't actually a "duration" unless a number is given (or "Until dispelled"); i.e. until a period of time passes to make "duration" meaningful.

From PHB, pg. 203:

Duration

A spell's duration is the length of time the spell persists. A duration can be expressed in rounds, minutes, hours or even years. Some spells specify that their effects last until the spells are dispelled or destroyed.

Instantaneous

Many spells are instantaneous. The spell harms, heals, creates or alters a creature or an object in a way that can't be dispelled, because the magic only exists for an instant.

Further, Xanathar's Guide to Everything (pg. 77) has this to say about Simultaneous Effects (thanks to @V2Blast for suggesting this):

If two or more things happen at the same time on a character or monster’s turn, the person at the game table — whether player or DM — who controls that creature decides the order in which those things happen. For example, if two effects occur at the end of a player character’s turn, the player decides which of the two effects happens first.

This also suggests that both fireballs should be resolved separately, making separate saving throws and damage rolls as though they did not happen simultaneously, but rather one after the other as though they happened a fraction of time apart.

Finally, the Essentials Kit includes some of these rules, but with a slightly different wording (thanks @Medix2). In the Essentials Kit Rulebook, on p. 3, under the section "Effects with the Same Name Don't Stack":

But when two or more effects have the same proper name, only one of them applies while the durations of the effects overlap (a duration is a time span of 1 round or more).

Durations is mentioned again, as in the PHB quote I included earlier in this answer (from p. 205), but in this case, what counts as a duration is made explicit; "a duration is a time span of 1 round or more". This also points to Instantaneous not being a real duration, further suggesting that both fireballs would deal damage.