Yeesh, The Nightingale and this in the same night? What a smorgasbord of unpleasant debauchery I unintentionally planned for myself!
This is a solid slow-burn horror that could be considered part of the "New French Extremity" wave of the 2000s if it weren't Belgian (eh, almost the same, right?!). I found the set-up to be quite effective, but it started to lose me a bit as the depravity began to escalate. Traces of dark humor were much appreciated, including an out-of-nowhere dance scene that rocked my damn mind.
Overall, if French-language 2000s horror were a mountain, Martyrs is the apex, High Tension is the dirt at the bottom, and Calvaire is the rest stop you take in the middle.