The Collaborative International Dictionary of English v.0.48:

Torturous \Tor"tur*ous\ (t[^o]r"t[-u]r*[u^]s), a.
   Involving, or pertaining to, torture. [R.] "The torturous
   crucifixion." --I. Disraeli.
   [1913 Webster]

WordNet (r) 3.0 (2006):

torturous
    adj 1: extremely painful [syn: agonizing, agonising,
           excruciating, harrowing, torturing, torturous,
           torturesome]

Moby Thesaurus II by Grady Ward, 1.0:

41 Moby Thesaurus words for "torturous":
   acute, afflictive, agonizing, atrocious, biting, consuming,
   cramping, cruel, desolating, distressing, excruciating, gnawing,
   grave, griping, hard, harrowing, harsh, heartbreaking,
   heartrending, heartsickening, heartwounding, hurtful, hurting,
   painful, paroxysmal, piercing, poignant, pungent, racking, rending,
   severe, sharp, shooting, spasmatic, spasmic, spasmodic, stabbing,
   stinging, tearing, tormenting, torturing