The narrative text for the Elder Brain from Volo's contains this passage (emphasis added):
When an elder brain infiltrates a mind, it alters the creature’s perception and deceives its senses, causing it to see, hear, touch, taste, or feel reality according to the elder brain’s intent. From across great distances, it implants subconscious suggestions or subtly influences dreams to compel creatures toward a course of action that benefits its grand plan.
We have just defeated a party of mind flayers and taken back something that the Elder Brain is likely to think very important.
We think the mind flayer party was out of range communication range of the Elder Brain, so I think we are more than the five miles specified by the EB's Creature Sense feature.
I am wondering about the "great distances" in the narrative above. Is that referring to the 5 miles, or is there reason to believe EBs can infiltrate minds at a greater distance? I think it must be referring to the 5 miles, but maybe it means something else, and I'm missing it.
I'm interested in RAW here, as a player, just to understand the monster better, and lore from previous editions would be fine as well. Our DM pretty liberally steals and splices and changes where he wants, but we pretty frequently discuss what sort of information (or misinformation) is available to researchers so likely if there's anything useful I will discuss it with him.
Best Answer
The only mechanical ability the Elder Brain has that remotely reflects the description of altering a creature's perceptions, implanting suggestions, and subtly influencing dreams is the Sense Thoughts power, which lets the Elder Brain read the target's thoughts and emotions, and
Sense Thoughts works only on creatures the brain has a psychic link to, and the psychic link breaks if you get more than 5 miles away, so it seems clear that this is indeed the 'great distance' it's talking about.