1) Atrocious 2) Bitter 3) Dolorous 4) Evil 5) Flagitious 6) Heartbreaking 7) Heartrending 8) Heinous 9) Important 10) Sorrowful 1) Critical 2) Dangerous 3) Dire 4) Distressing 5) French word used in English 6) Grave 7) Heartrending 8) Life-threatening 9) Like serious errors 10) Requiring serious thought 11) Rueful 12) Serious 13) Severe 14) Shockingly brutal or cruel 
• (a.) Characterized by great atrocity; heinous; aggravated; flagitious; as, a grievous sin. • (a.) Full of, or expressing, grief; showing great sorrow or affliction; as, a grievous cry. • (a.) Causing grief or sorrow; painful; afflictive; hard to bear; offensive; harmful.
Griev'ous adjective [ Old French grevous , grevos , Late Latin gravosus . See Grief .] 1. Causing grief or sorrow; painful; afflictive; hard to bear; offensive; harmful. « The famine was grievous in the land.» Gen. xii. 10. « The ...
causing grief or pain serious dire grave 
[adj] - causing or marked by grief or anguish
heartbreaking adjective causing or marked by grief or anguish; `a grievous loss`; `a grievous cry`; `her sigh was heartbreaking`; `the heartrending words of Rabin`s granddaughter`