I dont think the patronus kills them.... but fear gives them power and so do the souls that they suck out... right? so what happens if they lack those two things? human souls and human fear..... hmmm boggarts also feed off of fear... HMMMMMMM maybe im confusing them....

Dementors arent even IN Fantastic beasts and where to find them......
I wonder what they are

Alicia casts a dark look about the room wondering what these horrors could be...