Dark and dismal as of the rivers Acheron and Styx in Hades

ETYM Latin daemoniacus, from daemon; cf. French démoniaque. Related to Demon.
Resembling or suggesting possession by a demon; SYN. demoniacal.

Demonic, satanic; possessed by a demon

ETYM Latin daemonicus, Greek daimonikos.
Extremely evil or cruel; expressive of cruelty or befitting hell; SYN. diabolic, diabolical, fiendish, hellish, infernal, satanic, unholy.

Demonic.

Damned, confounded

Of, relating to, or characteristic of the devil; devilish

Of, relating to, or characteristic of the devil; devilish

ETYM French infernal, Latin infernalis, from infernus that which lies beneath, the lower. Related to Inferior.
1. Being of a lower world of the dead.
2. Characteristic of or resembling hell.
3. Relating to or inhabiting hell.

Diabolically, devilishly, fiendishly (also daemonically)

Someone who takes the worse side just for the sake of argument.
1. A Roman Catholic official whose duty is to examine critically the evidence on which a demand for beatification or canonization rests
2. A person who champions the less accepted cause for the sake of argument

Mischief, devilish actions