OK, so the locals decided to build this church in the middle of a graveyard. While building it, they had to clear a few graves. Unfortunately, the spot they chose was right over a mass grave containing the remains of some 40,000-70,000 dead people. After they finished the church, they decided they'd better do something with all the bones they'd dug up, so put them in an ossuary underneath the church. They gave a local wood-carver the job of arranging them artistically.
If you want to see chandeliers made of dead people, coats of arms made of dead people, writing on the wall made of dead people, vaulting decorated with dead people, candelabras made of dead people, tasteful wall ornaments made of dead people and piles and piles of dead people arranged neatly, this is the place to go.
Oh, and don't put your foot on the step before the pile of dead people in the back left-hand corner: when I did that, it set off an alarm (sigh).