Penny Dreadful2014

[the Cut-Wife speaks to Vanessa in the demonic language, teaching Vanessa]

Joan Clayton:
You must remember such incantations are dangerous. And you must never speak the Devil's language idly. Let this language not become easy in your mouth or soon it will no longer be your mouth but his. And it will tell only lies.

Vanessa Ives:
For he is the Father of Lies.

Joan Clayton:
If you believe in God, better you pray with all you got in you. Only if all else fails, you speak the Devil's tongue. But mark, girl, it's a seduction and before you can blink twice it's all you can speak, and so does a Daywalker become a Nightcomer.