It's a horror movie i think from the 80's or 70's, i saw it on TV when i was a little kid, about 15 years ago; please forgive me if i remember something wrong, it was a lot of time ago!

I just remember some scenes:

Some people studying something strange in the dungeon of a castle or something like a cellar.

A girl scratching her back with a piece of metal hanging out of the wall, then slowly all the back becomes swollen / rotten, and she turns into some kind of monster.

something like a demon, or a blody, skinless rotting creature slowly turning all the people into monsters themselves, or something like that.

An ending scene where a character suddenly wakes up in his bed from a nightmare, turns his head and find himself staring at a horrible monster looking like a skinned corpse.

Hope this helps! :D