hauntingly

Example:He heard the sound drifting in spectrally, eerily clear, from beyond his window.

Definition:having the appearance of a ghost or being ghostlike; related to spectral phenomena or the ghostly quality of something.

Example:The old man crouched in the corner of the room, watching with ghoulishly sharply-defined eyes as the others passed by.

Definition:characterized by a morbid, unpleasant, or disturbing quality, as in a ghoul; more specifically, in an unnatural or inhuman way.

Example:The director wanted the monologue to be delivered in an unforgettably emotional way that would leave the audience talking about it for days.

Definition:in a way that leaves a lasting impression or memory, often in a clearly unpleasant or disturbing manner.