The character theme for a villain associated with some of the game's most obnoxious boss battles. I usually know how far a new player is into the game by how much they wince when this theme plays.
The boss theme for the mostly optional Fiends, which are easily some of the nastiest fights in the game if you don't know what you are doing. Did I mention one of them appears after you use a save point,, any save point in the game?
Let me set the picture for you, you enter into Lea Monde's Undercity and as the screen loads, you hear a church bell ring in the distance jut as this theme kick in and is accompanied by the sound of children laughing, which are actually these really speedy marionette dolls that rush you in great numbers to murder you. This area is also filled with all the enemies with instant death magic, in a game with no defense against it. This is not a pleasant place.
Probably the one part of Bloodborne that is legitimately designed to be kind of freaky. Werewolves, Bloodsuckers, near total darkness and then the E.T. squad followed by something straight out of Lovecrat's own nightmares. This is the theme to that whole place.