grievous vs monstrous

grievous and monstrous both are adjectives.

Word	Noun	Adjective	Verb	Adverb
grievous	No	Yes	No	No
monstrous	No	Yes	No	No

As adjectives, grievous and monstrous are synonyms defined as:

Other synonyms of grievous include atrocious, flagitious.

grievous (adjective)	monstrous (adjective)
causing fear or anxiety by threatening great harm	shockingly brutal or cruel
shockingly brutal or cruel	abnormally large
of great gravity or crucial import; requiring serious thought	distorted and unnatural in shape or size; abnormal and hideous
causing or marked by grief or anguish