Can I modify days/months of memories using combination of "Dream" and "Modify Memory"?

Question

I'd like to modify a long period of the memories of the target.

Is it possible? Can I do it better? What are the problems with this method?

The algorithm:

timeline: Past-(Fake Past)-Present

The goal: Make the target think, that long fake past is real. Fake past does not contradict the past.

1. Wait till the target is asleep.

2. Cast Dream. During "8 hours" of the dream, I, as a Messenger, replicate the core events of Fake Past. ("The messenger can also shape the environment of the dream, creating landscapes, objects, and other images."). I cannot be 100% sure that the target will follow my plan, but let's assume this is not a problem.

3. The last segment of a dream would be the fake waking up and recalling the past events.

4. I cast Modify Memory to remove the memory about the real waking up.

I hope that "Its mind fills in any gaps in the details of your description" property of Modify Memory will help me, because I do not replicate all events during the dream. Only the core.

If it's not the case, I still can use my 8 hours of fake memory instead of 10 minutes. This is not months I hoped for, but good nonetheless.

Answer 1

Probably No

You can certainly give the target a dream in which they relive things that happened in the past months. You probably cannot have them relive three months' subjective experience in ten minutes, but that's up to your DM.

You can probably convince your target that a dream was real. The spell doesn't explicitly say you can do this, but it seems in line with other things the spell allows. Your DM will probably allow this.

But your modify memory spell will only let you affect the version of the events that your target has just experienced. The original memories will remain intact. So now your target thinks: "Okay, I went to the castle a month ago and I saw a creepy wizard looting the treasury. But then I also went to the castle five minutes ago and I saw a goblin looting the treasury and the creepy wizard was trying to prevent it. Also, how did I get back from the castle treasury to my bed in the span of five minutes? That's weird."

The modify memory spell notes that weird inconsistent memories may be ignored by the target, and I think that's likely to happen here.

It's not clear how the dream spell is helping you here. If you want the target to think they went to the castle, you can just tell them that using modify memory; there's no need to make them dream it first.

If you upcast the modify memory spell, you can affect the target's original memories without bothering with dream.