Fantastic Four Fight All-Powerful Villain
Some consider this first appearance of Diablo to be one of the great issues of Fantastic Four. I disagree.
There's a lot of nonsense here that's not explained right from the get go - why was the forest attacking the FF? How was it Diablo's voice could be heard all the way at the Baron's castle on this night to influence the Thing, but never before was it able to reach the Baron or anyone else. etc. etc.
The locals call the villain 'Diablo' (don't ask why Romanians are speaking Spanish) who has pretty much limitless (if temporary) powers. Which makes him another boring all-powerful foe that somehow makes some crucial mistake, or (as in this case) the Fantastic Four just plain get lucky (a spell wears off too soon).
There's other stuff like Sue Storm's powers being inconsistent in this issue, etc., but by the time we get to that, any intelligent reader has probably already checked out.
Woefully average - which is worse than you think because it's being judged by the yardstick of it's time.