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"
Sinister; evil; like a fiend.