"The Third Testament" in English, that's it, yes.
In this sequence, Marbourg and Elisabeth (our two protagonists) are sneaking down a booby-trapped chimney in order to discover a critical, lost, Christian document that refers to the location of the fabled Third Testament, one of the great undiscovered documents in Early Christian literature. (haha, take that Nag Hamadi & Qumran scrolls!) (eh, sorry, I guess that was a bit rude)
NOTE: Either zoom-in or right-click-open all these five images. I've uploaded them in the highest quality I can, but you'll need to do the former to see them properly, I think.
So, the story is about... wow... it's a deep story. But it's an absolute THRILLER at the same time, and I sure don't see that very often in terms of philosophical / historic / mythological works. Anyway, it's about the hunt for a certain lost tome of Early Christianity, which in turn reveals an absolutely breath-taking figure in the religion, still alive... still jonesing for... well, let's just say some BIG THINGS. oO
Personally, this is one of those that I'd call a "flawed-masterpiece." It has a few, couple hiccups in terms of pure-storytelling, and perhaps a bit of logistics-issues, but otherwise it's utterly fascinating, compelling work, with all kinds of interesting discussions and sub-discussions that it engenders, IMO. In plain English, this is an UNCOMMONLY good little, movie-like masterpiece.
(if it's not completely clear, Elisabeth winds up falling in to an underground lake, and it's *just* short enough of a drop as to not cause her physical harm)
More info and different perspectives here:
https://www.bedetheque.com/serie-23-BD-Troisieme-Testament.html
It’s really good. I loved reading it back in the day.