grievous vs life-threatening

grievous and life-threatening both are adjectives.

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

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

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

Other synonyms of grievous include dangerous, grave, serious, severe.

grievous (adjective)	life-threatening (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