Dementor

Dementors are Dark creatures in the Harry Potter continuum.

They cannot be seen by ordinary people, but everyone can feel the dark mood they press upon the area while they take in the good feelings and memories everyone had. Dementors reproduce through heavy mist.

They are capable (and more than willing) to suck a person's soul out of their body using the Dementor's Kiss

Dark!Evil!Harry Stu is often fond of having Dementors nearby.

Chocolate is especially helpful after having had to spend too much time near Dementors, IE, any amount of time whatsoever.