Synonyms for Horrendous:

adj.

awful (adjective)
horrific, unpleasant, alarming.
dreadful (adjective)
grievous.
frightful/frightening (adjective)
direful, fearsome.
ghastly (adjective)
frightening.
heinous (adjective)
odious, outrageous.
horrendous (adjective)
dread, dire, horrible, dreaded, dreadful, frightful, appalling, harrowing, ghastly, fearful, awful.
horrible/horrendous/horrid (adjective)
disagreeable, shocking, cruel, terrifying, offensive, terrible.
painful (adjective)
cutting, odious, unbearable, bothersome, crushing, irritating, burdensome, chafing, arduous, painful, Besetting, excruciating, aching, Cramping, uncomfortable, trying, sore, harrowing, aggravating, wrenching, grievous, severe, stabbing, miserable, bitter, inflaming, cruel, woeful, acute, withering, Torturing., annoying, piercing, anguishing, biting, distressful, offensive, Gnawing, tormenting, hurtful, grueling, troublesome, stinging, agonizing, unpleasant, insufferable, smarting, Irksome, throbbing, oppressive, disagreeable.

n.

outrageous, taboo, indecent. frightful, good, appalling, shocking, ghastly, horrible. horrendous (noun)
direful, frightening, fearsome, alarming, awful, dreadful, dire, fearful, dread, terrible, horrific, dreaded.
horrible (noun)
poor, terrifying, frightful.

Other synonyms:

appalling. shocking, taboo. horrible. ghastly. Other relevant words:
awful, direful, frightening, horrible, appalling, horrific, taboo, dreaded, ghastly, dreadful, alarming, dire, fearful, fearsome, outrageous, terrible, frightful, terrifying, indecent, dread, shocking.