The 'Pretty Little Liars' Canadian promo is here for 'Bite Your Tongue' (episode 4x17), and it reveals something quite creepy that A does to one of the girls. Seriously, if this happened to us in real life we would have nightmares forever.

Proceed below to find out what it is. Just beware of all those spoilers.

The promo opens with Hanna revealing to the girls that while at the dentist she is attacked and treated by A -- which is made even more horrifying when later on Spencer pulls what appears to be a piece of paper out of Hanna's tooth.

A has never been more terrifying. Or confusing, for that matter.

The video also features Emily frightened inside the high school (which loses power), prompting her to grab scissors as a weapon, a flashback to when Ali was considered alive and a a voice-over referring to the devious Mona as "the devil."

Catch 'Bite Your Tongue' on Jan. 28, 2014, on ABC Family at 8PM ET to see how much more the plot can thicken. We're guessing pretty thick.