D&D 5E The most obvious spell, and it's always missing

[The Crimson Binome]

Harry, Ron, and Hermione pulled it off pretty easily and consistently in Deathly Hallows.

From what I recall, that was a short-term (hour or less) effect which affected random muggles. It's way easier to make someone forget what they're doing, or follow a reasonable alternative, than to force them to do something specific for 8 (or 168) hours. And it was only ever a very mild compulsion, that could have been ignored by anyone who was actually determined to find them, which is why their main defence was to keep on the move.

Also, as students in what would have been their seventh year, they were just about at the peak of the spells available to them.

Also also, that magic system is complete shenanigans, and should not be taken as a serious example of how magic would work in a reasonable setting. I mean, seriously, the universe cares that you pronounce wingardium leviosa with exactly the right inflection, or else it won't work? Was it a toddler who came up with that incantation?

 

[Mirtek]

D&D has had a lot of spells over the decades. D&D is also a resource driven game, and has always had resources that reset after a certain time period (whether that be once per day, once per encounter, once per rest, etc).

But you what spell is missing, that is super obvious? An avoidance one, party based at least. There are tons of alarm spells, and spells like rope trick, and other spells meant to do some damage to anything that crosses a borderline, etc. But 90% of them don't actually stop your rest from being interrupted with the exception of spells like rope trick. Your rest is interrupted and you have to start all over again. I would think that some low level wizard would quickly devise a spell that instead of tripping an alarm, or sending a blast of energy or arrows into intruders, would instead cause the intruder to just walk away. Something like a WIS save or turn around and leave, and avoid the area for the duration. I'm not aware of any spell like that.

The problem with such a silent spell is that if the intrude makes his save, he gets to slit your throat while you sleep. Better to have your rest disturbed than your life ended

 

[The Crimson Binome]

How do you know the weight of magic required? Perhaps it is an ounce, or a gram of magic?

The outcome of the effect must be proportional to the energy expenditure. That's why you can't have a third-level spell that sends someone to an alternate dimension populated entirely by piranhas for one round, even if the damage is equivalent to what you'd get from Fireball, because dimensional travel (and retrieval) requires more work than just setting stuff on fire.

 

[TwoSix]

From what I recall, that was a short-term (hour or less) effect which affected random muggles. It's way easier to make someone forget what they're doing, or follow a reasonable alternative, than to force them to do something specific for 8 (or 168) hours. And it was only ever a very mild compulsion, that could have been ignored by anyone who was actually determined to find them, which is why their main defence was to keep on the move.

Also, as students in what would have been their seventh year, they were just about at the peak of the spells available to them.

Also also, that magic system is complete shenanigans, and should not be taken as a serious example of how magic would work in a reasonable setting. I mean, seriously, the universe cares that you pronounce wingardium leviosa with exactly the right inflection, or else it won't work? Was it a toddler who came up with that incantation?

1) I'm pretty sure they put up that hex when they camped during the night, and it lasted all night. Remember, it kept some Death Eaters from discovering their camp in Deathly Hallows Part 1 when they were literally right next to it.

2) A serious example of how magic would work in a reasonable setting? Really? Compared to the system where bat poop creates megajoules of thermal energy?

 

[Jaelommiss]

Perhaps a Glyph of Warding combined with Suggestion: "You forgot something important at home. Go get it!" would produce the desired effect. At 100gp nightly and only affecting a single target it is very costly, but perhaps it could be built on from there?



Jaelommiss' Bewitching Ward
3rd-level enchantment

Casting Time: 1 hour
Range: Self (20-foot radius)
Components: V, S, M (a splash of spiced wine)
Duration: 8 hours

You pour a small amount of spiced wine on the ground at your feet, where it spreads out over the floor in a 20 foot radius before gradually vanishing. For the duration of the spell, this area is warded against unwanted intrusion.

Creatures who enter the warded area must make a Wisdom saving throw. Creatures who have a pressing reason to stay (for instance, a creature wondering "why is there a wizard sleeping there in the storeroom") make the save at advantage. On a failed save, the creature remembers urgent business that it has elsewhere. The exact nature of this business will depend on the creature's nature and be determined by the DM, but never requires more than one hour to complete. The creature then pursues that course of action to the best of its ability, freeing itself from this spell upon its completion. Whether the saving throw succeeds of fails, you are immediately awoken and made aware that a creature triggered this ward.

Creatures immune to being charmed are immune to this spell.

At Higher Levels. When you cast this spell using a spell slot of 4th level or higher, the radius of the warded area increases by 10 feet for each slot level above 3rd.



Notes: I was determined for it to be 3rd level since that is the same as L. Tiny Hut and Glyph of Warding, both of which were drawn upon as inspiration for this spell. I was unsure whether to make it a ritual, but thought that being able to increase its radius would be helpful, and therefore would require expending slots. Its seemed strange that the spell would work even if the victim could clearly see the party within the area, and granting advantage on the save seemed an appropriate way to balance that. Adding a 1 hour casting time in addition to advantage on the save seemed like a good way to prevent it being used offensively. I considered a costly material component, but was unable to decide on a fair price. Since it is likely to be used each and every night, putting a cost on it seemed unnecessarily punitive, and could even make the spell impossible to use in games with longer travel times.

 

[The Crimson Binome]

1) I'm pretty sure they put up that hex when they camped during the night, and it lasted all night. Remember, it kept some Death Eaters from discovering their camp in Deathly Hallows Part 1 when they were literally right next to it.

I haven't seen the movies. I've read the series twice, and many of the supplemental materials. It's possible that I have some details incorrect, but it didn't seem like the warding spells were sufficient to prevent a concerted search effort, which is why they had to keep moving.

2) A serious example of how magic would work in a reasonable setting? Really? Compared to the system where bat poop creates megajoules of thermal energy?

The nice thing about the D&D system is that it's vague enough for us to fill in the blanks with whatever makes sense to us; there's an incantation (probably in a fictional language), and the guano is consumed at some point, and that's all we really know. To contrast, Harry Potter is set in a world where Greek and Latin and English are all real languages (so there's no TranslationConvention at work), and the incantation to make something float is "Wingardium Leviosa".

 

[Hussar]

The outcome of the effect must be proportional to the energy expenditure. That's why you can't have a third-level spell that sends someone to an alternate dimension populated entirely by piranhas for one round, even if the damage is equivalent to what you'd get from Fireball, because dimensional travel (and retrieval) requires more work than just setting stuff on fire.

Umm, Rope Trick is a dimensional travel spell that's 2nd level. Blink is 3rd. Banishment is 4th. Dimensional travel is not exactly hugely more difficult than fireball.

 

[TwoSix]

I haven't seen the movies. I've read the series twice, and many of the supplemental materials. It's possible that I have some details incorrect, but it didn't seem like the warding spells were sufficient to prevent a concerted search effort, which is why they had to keep moving.

Sure, but my main point is that it WAS sufficient to cover an overnight period, which was the point of the original discussion, the relative difficulty of an illusion/compulsion that last for a few minutes compared to several hours.

The nice thing about the D&D system is that it's vague enough for us to fill in the blanks with whatever makes sense to us; there's an incantation (probably in a fictional language), and the guano is consumed at some point, and that's all we really know. To contrast, Harry Potter is set in a world where Greek and Latin and English are all real languages (so there's no TranslationConvention at work), and the incantation to make something float is "Wingardium Leviosa".

Sure, that's fine. I was only noting that you seemed to have some implicit assumptions about how magic "would" work in order to be "reasonable". Those tacit assumptions usually draw my attention as I rarely agree with them.

 

[The Crimson Binome]

Umm, Rope Trick is a dimensional travel spell that's 2nd level. Blink is 3rd. Banishment is 4th. Dimensional travel is not exactly hugely more difficult than fireball.

Rope Trick creates a pocket dimension, Blink lets you waver across the border between this plane and the adjacent one, and Banishment takes a minute to send a creature on a one-way trip to a plane to which that creature is already connected. A back-and-forth six-second trip to the piranha dimension would require more work than any of those.

I'm not saying that everything is perfectly quantifiable in terms of energy expenditure, but relative energy requirements follow obvious patterns. A long-duration no-concentration compulsion effect would require significantly more power behind it than a simple charm spell would.

 

[Prism]

What you want is AD&D Unearthed Arcana, Tempus Fugit, 5th level illusionist spell