The Cave of Crimson Heart by Eugene Ch.

48. After setting up puzzle traps with a bait, 'he' made Noonsius explain funny rule
which designed to make victims turn away their eyes.
When victims who finally had passed the dance lose their tension on last step, 
their heads were cut off by boomerangs. Then Noonsius-san get hearts from their cooling bodies.  
/Eh, Where is Noonsius-san? / He seems to run away when boomerangs flied.