life-threatening vs grievous

life-threatening and grievous both are adjectives.

Word	Noun	Adjective	Verb	Adverb
life-threatening	No	Yes	No	No
grievous	No	Yes	No	No

As adjectives, life-threatening and grievous are synonyms defined as:

life-threatening and grievous: causing fear or anxiety by threatening great harm

Other synonyms of life-threatening include dangerous, grave, serious, severe.

life-threatening (adjective)	grievous (adjective)
causing fear or anxiety by threatening great harm	causing fear or anxiety by threatening great harm
	shockingly brutal or cruel
	of great gravity or crucial import; requiring serious thought
	causing or marked by grief or anguish