Here are some synonyms for fearsome:
* Terrifying
* Horrifying
* Dreadful
* Awful
* Formidable
* Menacing
* Threatening
* Daunting
For example, you might say:
* "The lion's roar was fearsome."
* "The storm looked fearsome, with lightning and thunder."
* "The knight's armor was fearsome, designed to intimidate his enemies."