WordNet sense 1 (hellish):

stygian

WordNet sense 2 (of or relating to or characteristic of Hades or Tartarus):

Hadean, Plutonian, Tartarean

WordNet sense 3 (dwelling beneath the surface of the earth):

chthonian

WordNet sense 4 (extremely evil or cruel; expressive of cruelty or befitting hell):

demonic, diabolic, diabolical, fiendish, hellish, satanic, unholy

WordNet sense 5 (morally bad or wrong):

evil

WordNet sense 6 (expletives used informally as intensifiers):

blame, blasted, blessed, damned, darned, blamed, deuced, goddam, goddamned, blessed, goddamn

WordNet sense 7 (deserving a curse; sometimes used as an intensifier):

cursed, curst