forbidding

daunting, demoralizing, disconcerting, discouraging, dismaying, disquieting, distressing, disturbing, perturbing, startling, threatening, troubling, trying, unnerving

bone-chilling, creepy, eerie (also eery), weird

appalling, atrocious, awful, grisly, gruesome (also grewsome), hideous, horrid, macabre, monstrous, nightmarish

bleak, cold, hostile, inhospitable, inimical, unfriendly, unsympathetic

adamant, bound, determined, firm, intent, purposeful, resolute, resolved, steadfast, unflinching

fixed, hard, hardened, hardheaded, immovable, implacable, inflexible, ironhanded, mulish, obdurate, obstinate, rigid, self-willed, set, stiff, stubborn, unbending, uncompromising, unrelenting, unyielding, willful (or wilful)

immutable, unchangeable

black, cheerless, dark, gloomy, glum, joyless, melancholic, moody, morose, sulky, sullen, surly

brooding, grave, humorless, melancholy, serious, sober, sobersided, solemn, somber (or sombre), staid, unsmiling, weighty