I'm new here so I'm sorry if this has been noticed before and addressed, but I was reading the official FAQ/Errata document for Talisman. On Page 3 we have this:

Psionic Blast

Q1: If a character casts the Psionic Blast Spell while fighting the Pit Fiends, does it only affect a single battle against one of the Pit Fiends?

A: Yes. Each Pit Fiend is considered a separate battle.

Good to know...but the rules of Talisman (page 19 of the main rule book) state that no spell may affect any of the creatures in the Inner Region. Was this changed at some point?

Psionic Blast affects the caster (boosting his Str), it doesn't directly affect the Pit Fiends, like Spells such as Fireball or Finger of Death would. Same deal as to why you can use Invisibility to Evade or Psionic Blast to boost vs a character with Amulet. Amulet makes him immune to be affected by spells, but both of those affect you the caster, not the target.

brilliant! thanks!

True but you cannot use invisibility to evade monsters in the inner board.