The only thing I'm confused about is if the target isn't asleep, does the time until they do count against the duration? Or does the duration only start once the dream does?

So it doesn't say clearly in the spell description, if the Caster acts as the Messenger, does the "trance" they enter last for the duration of the dream spell? So it's a one minute cast time and you then enter an 8 hour trance?

And if the Target fails their wisdom saving throw, it says that the monstrosity haunts their dreams, so you could potentially say your 10 spooky words, induce the nightmare effect of this spell, gtfo and the nightmare will carry on even if you ended the spell?

Currently playing a Warlock and I'm not sure if this is a good spell to use if you're out adventuring if you want to torture people.
Was thinking of combining it with a Bag of Tricks, so I can call an animal, cast the spell and then have them on standby till my Target is a sleep and then have them be the monstrous nightmare... It doesn't say in the spell that the messenger needs to know any languages, and seeing as it's a "dream" I'm sure it's not too farfetched to say that animals can speak in the dreams??

It would be more reasonable to assume 8ish days.  Additionally, they would have to fail the exhaustion saves when they wake up.

Time to pull of an extraction of memory

My bard just used legend lore on the BBEG and discovered that she had a lover that she was forced to kill.  This led to her making a pact and becoming the evil monster she is.  Now I'm going to take Dream when I reach the next level and start having her dead lover haunter her dreams.

Quote from Mooxim >>

Since 6 points of exhaustion means death, couldn't you use this to assassinate anyone on the same plane of existence over the course of 6 nights.
Once the target has taken 3 points of exhaustion it would have disadvantage on saving throws against your further spells.
You could argue that the target attempts another long rest after the eight hours is up. In which case, you'd have to cast the spell again, up to 3 times a day.
So an 18th level spellcaster or 3 x 9th level casters could do it.

 or just one level 11 warlock

AL says you can only benefit from one Long Rest per 24 hour period, so if one is screwed you cant just try to Long Rest again 

Quote from Yhniwyadw >>

AL says you can only benefit from one Long Rest per 24 hour period, so if one is screwed you cant just try to Long Rest again 

I can see how you’re thinking that, but that’s not quite right. If a person’s short rest is screwed/interrupted before the full 8 hours* is up (for instance, they only get 5 hours of sleep before getting ambushed and casting a spell in defense), then they haven’t benefitted from a long rest at all and therefore can restart it. But if they get a full long rest and try to take another one (say, 15 hours later, for example), it wouldn’t work because they’ve already had a successful and complete long rest that day.

Hope that helps!

*or 4 hours for things like elves, etc.

What if I've only seen my target once, and I didn't clearly see his face and don't know his name ? Can I still enter his dreams ?

My knowledge of the target is basically "a child that was clinging to the BBEG" 

If an Elf is the messenger can they combine the Dream spell Trance with the Elf trait Trance to spend their 4 hour long rest in other characters' dreams?

I don't see why not, although I'm not sure whether or not the messenger can rest while they're the target of a spell like this. Even so, you can screw up their rest and get a full night's sleep before they even wake up from their nightmare! Hehehehe...

This spell does not cause exhaustion folks. It simply prevents you from recuperating from it (or regaining hit points or spell slots, or any of the other "benefits of a long rest".

The rules state that you must complete the long rest, sleep, and have access to food/water/etc. If you do, you gain the benefits of a long rest, and avoid picking up a point of an exhaustion. THESE ARE AN IMPORTANT DISTINCTION!!!

This spell does NOT PREVENT any of that. You still sleep. You still complete the long rest.

However, it does prevent BENEFITS from that rest, ie, recovery of lost/spent resources. No spell slots back, health recovery, or REMOVAL of a point of exhaustion.

But NOT picking up exhaustion is not a BENEIFT that is conveyed of long rest. It is what is PREVENTED by successfully completing the long rest, which still occurs. The requirements of the long rest are still met, but you gain none of the ancillary benefits of what it RESTORES, but not what it PREVENTS.

Now, this is still a great spell, but it doesn't cause exhaustion, at least not at my table. ;-)

YMMV, and of course everybody can interpret this as they wish. But I think the verbiage is fairly clear on this in the distinctions between what benefits are gained from a long rest, and what is prevented by not completing one to begin with.

Since the spell isn't concentration, can the messenger benefit from a long rest while under the effects of the spell?

How is this spell not available to Circle of Dreams druids?

YES THANKS just what i thought well they can use it at level 14 but im sorry circle of dreams dosnt get dream wtf

I shall use this spell to make Cobb from Inception

Why can't a Circle of Dreams druid cast this? Only at level 14 and on themselves? Make it make sense

Or a Warlock that takes enough Short Rests could do it at Level 9.

@TheRhineDude I agree with you for RAW, but I'd probably have the target roll a Con save for an exhaustion point still. 

It would make killing a person with the spell pretty hard and a long process where the target could very well figure out what is happening to them. Each successful Wisdom save would reduce the exhaustion by 1 and even a failed Wisdom save allows for a Con save to prevent the exhaustion. I could see it taking significantly longer than 6 days. 

Still a damn good spell to use the night before you attack a BBEG.

Going without a Long Rest

A long rest is never mandatory, but going without sleep does have its consequences. If you want to account for the effects of sleep deprivation on characters and creatures, use these rules.

Whenever you end a 24-hour period without finishing a long rest, you must succeed on a DC 10 Constitution saving throw or suffer one level of exhaustion.

It becomes harder to fight off exhaustion if you stay awake for multiple days. After the first 24 hours, the DC increases by 5 for each consecutive 24-hour period without a long rest. The DC resets to 10 when you finish a long rest.

Quote from TheRhinoDude >>

This spell does not cause exhaustion folks. It simply prevents you from recuperating from it (or regaining hit points or spell slots, or any of the other "benefits of a long rest".

The rules state that you must complete the long rest, sleep, and have access to food/water/etc. If you do, you gain the benefits of a long rest, and avoid picking up a point of an exhaustion. THESE ARE AN IMPORTANT DISTINCTION!!!

This spell does NOT PREVENT any of that. You still sleep. You still complete the long rest.

However, it does prevent BENEFITS from that rest, ie, recovery of lost/spent resources. No spell slots back, health recovery, or REMOVAL of a point of exhaustion.

But NOT picking up exhaustion is not a BENEIFT that is conveyed of long rest. It is what is PREVENTED by successfully completing the long rest, which still occurs. The requirements of the long rest are still met, but you gain none of the ancillary benefits of what it RESTORES, but not what it PREVENTS.

Now, this is still a great spell, but it doesn't cause exhaustion, at least not at my table. ;-)

YMMV, and of course everybody can interpret this as they wish. But I think the verbiage is fairly clear on this in the distinctions between what benefits are gained from a long rest, and what is prevented by not completing one to begin with.

that appears to be house rules, most others online and on dnd beyond state it causes exhaustion because of the last clause

”and prevents the target from gaining any benefit from that rest.” The benefit being the rest itself. So they will need to start a new long rest within same 24 hours to prevent exhaustion.

https://www.dndbeyond.com/posts/1215-dead-tired-how-exhaustion-works-in-dungeons#:~:text=The dream spell can indirectly,of exhaustion when it ends.

”The dream spell can indirectly cause exhaustion by denying the target the benefits of a long rest if they fail their Wisdom saving throw. ”

Eventually they will need to finish a long rest to remove new layers of exhaustion, since they would not get this benefit.

”Finishing a long rest reduces a creature's exhaustion level by 1, provided that the creature has also ingested some food and drink. Also, being raised from the dead reduces a creature’s exhaustion level by 1.”

lastly “ On a failed save, echoes of the phantasmal monstrosity spawn a nightmare that lasts the duration of the target's sleep and prevents the target from gaining any benefit from that rest.“

essentially

“performs no more than 2 hours of light activity, such as reading, talking, eating, or standing watch. If the rest is interrupted by a period of strenuous activity - at least 1 hour of walking, fighting, casting spells, or similar adventuring activity - the characters must begin the rest again to gain any benefit from it.”

in this case the nightmare which lasts 1minute to 8 hours is “strenuous”. The nightmare lasts the remaining duration. the nightmare failed save and damage needs to be at least a 1-2 hours before they wake up to cause stress. 

they need to finish a non-strenuous rest  within the 24 hour time.