It's a 2014 horror movie set in the Paris Catacombs. As you might be able to tell by the title, the framing device is esotericism, following a woman convinced that the philosopher's stone is hidden somewhere in the catacombs. It popped up in my Youtube recs as free with ads (read: totally free with adblocker) and I didn't have anything going on and vaguely recalled hearing a positive thing.
The movie uses found footage presentation with everything being seen from a handheld camera or the incredibly good yet microscopically small cameras installed on everyone's headlamps. Yet it doesn't really have a found footage feel to me. Lacking a "this camera was discovered in an abandoned tunnel" disclaimer at the start or a "dropped the camera and perished offscreen" ending or just... a certain level of mundanity I expect from found footage.
That said, it does make the movie feel appropriately claustrophobic with the camera perspective being in the group rather than outside it. And for the most parts the actors are looking at each other's faces and not at the cameras when there's a forehead-camera recorded conversation going on.
The horror worked me, specifically. For these supernatural kinds of horror I often prefer some mystery and unpredictability to keep me engaged. Things being over-explained or limited lessens the impact. So the unpredictability of what they encounter kept me hooked, and I liked the prevalence of odd sights.
Aside from one big "Oh you did not just do that c'monnnnnn" moment at the end the plot is mostly serviceable but let down for me specifically because it retread a lot of ground that I'm already tired of walking on.
Here there be spoilers
But overall I liked it. If I had seen it in theaters in 2014 I probably would've bought the DVD. And I mean I watched it for free so I can't complain. Also I have no will to complain.