Mercury's soul is in the church library. After playing with a knight BOSS, the wall next to it collapsed. The specific route is to go to the torture room first, then run the ghost lamp upstairs and then go to the atrium. (See the guide ...) You can see the tutorial.

After you get it, use the "ghost claw" on the rotating blade, which is L, then he floats, then splits him, splits him to rotate quickly, then installs L, and then you can blow it out. The attack is quite fierce, Mafia. In one second, the blood is thicker and it has the effect of flying.

By the way, when you hit the rotating blade with an L, never lock it. If you lock it, you will catch it, not hit it.