Detecting the Thoughts of Fiends causes Insanity?

[Li Shenron]

Only related thing coming to my mind is getting stunned when trying to Detect Thoughts on someone too clever for you

 

[Zappo]

There is a similar rule in the Planescape Planes of Chaos boxed set. It applies to demons there. I don't seem to remember a similar rule for other fiends, not in Planescape at least.

 

[ruleslawyer]

The rule you're looking for is indeed in the 2e boxed set Planes of Chaos, along with lots of other nastiness (the ability of Abyssal lords to twist divination spells to their own ends, etc.).

The major problem is that this is a powerful, arguably CR-altering ability. As such, it's a bit difficult to shoehorn into a 3e game. IMC, fiends have various other immunities and abilities beside the core ones, but I've had to do a lot of juggling with the CRs, especially on the lower-order fiends; by the time you're up at the horned devil (CR 16), adding a few extra abilities is fine, but dealing with the CR 8 and under beasties is hard.

 

[RuminDange]

For a 3E reference, if my memory serves me correctly since I don't have access to my books at the moment (work interferes too much with game ). Check the Probe Thoughts spell in Defenders of the Faith. I think it says something about reading the minds of creatures alien to the caster could cause problems, but no hard and fast rule.
Another place to look might be Mind Raid in R&R if you want to go outside the WOTC books. But I don't recall that one saying anything like that, but it has been a while since I read that one.

RD

 

[BOZ]

i had thought that i had seen this rule in a 1E book before, but i couldn't mind it. and yes, the planescape reference does sound familiar.