First adjective for WCW: a: fierce, cruel, terrible b: sinister, malevolent c: very destructive.
Using this word to get the full effect is tricky. It's not common, it's on the archaic side, it's on the purple side, I suspect. Going to call this a twenty-five cent word. Best used with a handful of ten-cent words to prep the reader IMO.
More specific than:
Scary, horrible or any of the words listed in its definition. In my mind, this adjective is for slavering beasts that chase you through the woods at midnight.
Merriam-Webster lists some synonyms, most of which are lower-impact than fell. But there are a few that I think it shares that peculiar balance of archaic purple-ness. Baleful. Mortal, as an adjective -- a mortal wound. Stygian belongs in that group too. Abyssal. Why do these words get a bad rap? Because they're a bit archaic? Were over-used in the past? How long does it take a word to recover from over-use? (because we need awesome back, IMO).
What comes to your mind? Post a sentence using it? (can you do it without turning completely purple?)