Satan
Old Nick
Devil
Lucifer
Beelzebub
the Tempter
Prince of Darkness

Jack and Ma refer to the bad man as Old Nick.

Old Nick's account could not be settled in London.

And the idea of the author being the Old Nick is a really interesting way of looking at it.

Old Nick refuses, afraid that the boy will inform the authorities about Room.

They go before Old Nick and demand to inspect his demons, as custom allows.

Old Nick just sits in his suite, smoking and drinking.

"Old Nick himself must have been down this way!"

Old Nick has never gotten a good look at Jack or even really wanted one.

Old Nick brought down one arm in an emphatic gesture.

He acts as if he'd seen Old Nick himself.