Fiendish
WordNet
adjective
(1) Extremely evil or cruel; expressive of cruelty or befitting hell
"Something demonic in him--something that could be cruel"
"Fires lit up a diabolic scene"
"Diabolical sorcerers under the influence of devils"
"A fiendish despot"
"Hellish torture"
"Infernal instruments of war"
"Satanic cruelty"
"Unholy grimaces"