Extremely Difficult or Challenging; Devious
This page is the CodyCross answer for the clue below from Underground Worlds, Group 2166, Puzzle 1 (Peridot and Pyrite).
Question is: Extremely Difficult or Challenging; Devious
and the answer is: FIENDISH
Word Meaning:
FIENDISH — Sinister; evil; like a fiend.
Definitions from Wiktionary.