Synonyms:
* General: gloomy, glum, morose, moody, dour, somber, surly, withdrawn, brooding, dismal, disgruntled, downcast, forlorn, ill-humored, irritable, pessimistic, unhappy
* More intense: furious, savage, malicious, spiteful, resentful
* More formal: melancholy, lugubrious, despondent, dejected, gloomy
Related Words:
* Related to mood: depressed, anxious, stressed, worried, uncomfortable, miserable, unhappy, sad
* Related to behavior: uncooperative, silent, stubborn, resistant, aloof, hostile, unfriendly, grouchy, crabby
Figurative Language:
* Metaphors: a storm cloud, a dark shadow, a leaden weight, a heavy heart
* Personification: The sullen sky hung low over the city. The trees stood sullen and silent.
Wordplay:
* Antonyms: cheerful, happy, joyful, bright, optimistic
* Homophones: sullen (adjective), sullen (verb - archaic)
Note: The specific variation you choose will depend on the context and the nuance you wish to convey.