"What a devilishly simple key it was, after all. For those who follow me on my journey of discovery, apply this key to the runes on the Amulet: subtract three, then read. There are two exceptions, but not troublesome ones."

Being a TSR module (i.e. written for 12 year olds), it really is as simple as it sounds. Take each letter, back up three letters in the alphabet, and voila. Nonetheless, this intrepid chronicler got it wrong between gaming sessions and spent many weeks wracking his brain for an answer.

Amulet 1: wkh  idfh  ri  wkh  ilhqg  grhv  pruh  whdq  ghyrxu 
Translation: the  face  of  the  fiend  does  more  than  devour 
 
Amulet 2: zlwk  wkh  ohdvw  ri  p(y)  ivrp  wlv  wkh  jds  wr  arzhu 
Translation: with  the  least  of  m(y)  form  tis  the  gap  to  power