Is there a way to scan someone's memories while they sleep without them knowing?

Question

In my campaign, I want to have an NPC group that is gathering intelligence by kidnapping people while they sleep, scanning their memories, and returning them before morning. I know that I could just rule that they can do this, but I wanted to see if there was a spell that could be used for the purpose.

So far, I have looked at detect thoughts and modify memory, but neither of them has the exact effect I'm looking for. A related question asked about encode thoughts suggests that spell could be used, but it's also from the Guildmaster's Guide to Ravnica (p. 47), and my campaign doesn't take place in the setting.

Are there spells, class features, or racial features that can do what I'm trying to accomplish? I would prefer to look only at PHB and XGtE, but if there are spells, class features, or racial features in other source books, I would be happy to consider those as well.

Answer 1

Use Detect Thoughts guided by an interrogation, followed by Modify Memory

You've stated that that neither Detect Thoughts nor Modify Memory does what you need, but what if you used both of them together? First kidnap the target in their sleep. Then wake them up and have one person cast Detect Thoughts and focus on reading the target's thoughts while another person interrogates the target. Whether or not the target lies in their answers, the Detect Thoughts spell will likely get the information you want. Waking the target up and interrogating them is necessary because even if you probe deeper, Detect Thoughts can only tell you what the target is thinking right now, so if they aren't thinking about what you're interested in (or dreaming about it, if they're asleep), you're out of luck unless you have a way to guide their thoughts toward the desired topic.

Make sure this whole process takes no longer than 10 minutes (the maximum length of a memory that can be modified). Then, cast Modify Memory and erase the target's memory of the whole process, instead making them remember being asleep the whole time. Then, use some means of putting them back to sleep before the Modify Memory spell ends. Since the spell makes them "incapacitated and unaware of their surroundings" for the duration, they will have no memory of being knocked out, in addition to having no memory of the interrogation. Finally, sneak the sleeping person back into their bed. When they wake up in the morning, as far as they know, they have slept all night uninterrupted.

Note: don't use Modify Memory to make them think it was all a dream, unless you want all the townspeople to compare notes and eventually realize that they're all having the same weird dream about being interrogated. Just erase the memory entirely.

Answer 2

If the objective is simply to find a RAW-compliant way to abduct a target, read their thoughts, and have them wake up the next morning unaware, then it sounds like a combination of both of those spells would be appropriate: the target might have detect thoughts cast on them during the night to probe for details, and then modify memory could be used to render them unaware that anything happened afterwards, either wiping the entire encounter from their memory or convincing them that it was a dream that happened while they were asleep.

Answer 3

Make one of the members of your NPC group a doppelganger, and have a nothic working for them as well.

A doppelganger is a monstrosity, but can take the form of a humanoid. Its Read Thoughts ability is similar to detect thoughts, but it can be used at will, costing no spell slots or components; and, unlike detect thoughts, the target has no way of knowing its thoughts are being read.

Like detect thoughts, Read Thoughts reads a creature's "surface thoughts" and does not probe its memories. However, the doppelganger's description does state that it can glean "a few scattered memories." In addition, the doppelganger has advantage on Deception, Intimidation and Persuasion checks on a creature whose thoughts it is reading. This can be used to its benefit when conducting interrogations.

A nothic is an aberration who was once a humanoid wizard. It is normally a solitary creature who skulks in the shadows, but it does covet magic items and will share its secrets with those who offer it something it wants. Its Weird Insight ability requires the target to make a Deception check which is contested with the nothic's Insight check. If the nothic's check prevails, it learns one "fact or secret" about the target.

Neither the doppelganger nor the nothic have a way to directly probe a creature's memories as such. But in combination, their abilities provide a plausible method by which your NPC group can gather intelligence from the minds of the locals.

Answer 4

Use sleep and detect thoughts and don't worry if they notice.

Sleep breaks if the creature takes damage or is shaken awake, but there's no reason to think detect thoughts will break it.

If they make the save to notice the scan, they'll most likely just have a weird dream. They can't try to fight off the scan because that's an action, and they're unconscious. If they wake up, just cast sleep again--it's only a first-level spell. They'll wake up in their own beds the next morning and convince themselves it was a dream.

If a handful of them do remember being abducted and probed by mysterious weirdos, well, we have real people who claim the same thing. They're generally dismissed as crackpots.

Answer 5

As the other answers have indicated Modify Memory would be a good option to cover your tracks after you have completed your interrogation. For what to do prior to that you have several options:

1. Detect Thoughts is a possible, but slightly dubious option. It can only read what they are thinking. If they are sharp enough they might be able to avoid thinking about what you are asking about. Keep in mind the rules for the spell states:

   If it fails, you gain insight into its reasoning (if any), its emotional state, and something that looms large in its mind (such as something it worries over, loves, or hates).

   If whatever you want know isn't one of these things, it won't help.

2. Dominate Person can force a person to obey your commands, so you could cast that on them then order them to tell you what they know.

3. Magic Jar might work, depending on how you interpret the spell. It gives you control of their body and you could rule that it gives you access to their memories as well.
4. Suggestion could be used to ask the person to tell you want to know.