Two answers in a crossword puzzle were MORTAL ENEMY and FATAL ERROR. The "revealing" clue was TRUTH KNOWN ONLY TO A FEW. What was the answer to the revealing clue, and how does it explain MORTAL ENEMY and FATAL ERROR?

HINTS:
#1: The answer to the revealing clue has two words.
#2: The second word is STORY.

Answer to the revealing clue:

Truth known only to a few = INSIDE STORY

Answer to how INSIDE STORY explains MORTAL ENEMY and FATAL ERROR: 

morTAL Enemy and faTAL Error 

Credit: Mike Peluso