Creepy Voice is a presence that follows Puppysmiles throughout her journey, looking to overpower and claim Puppysmiles body for her own.
Creepy Voice revealed she was once a part of Nightmare Moon, when Twilight Sparkle and her fellow bearers of the Elements of Harmony defeated Nightmare, Creepy Voice was a small fragment of her essence that managed to survive. Without a willing host, it was utterly powerless and unable to affect Equestria.
Creepy Voice found a suitable host in the innocent filly Puppysmiles. She watched Puppy as she stopped at various locales along Route 52, making friends and searching for her mother. She first speaks to Puppysmiles outside Rust Manor when she entered the Solaris Stable, Puppy names her Creepy Voice as she does with others who she has spoken to but not met, like SolOS or Watcher. Creepy Voice offers to provide aid if Puppy will give her control, Puppy is weakened due to being attacked by several robots in the Solaris Stable and agrees. Creepy Voice takes control, easily dispatching SolOS's robots and moving towards his A.I. core.
Creepy prepares to destroy SolOS, which Puppysmiles objects to, Creepy simply gloats about Puppysmiles being unable to stop her. To Creepy's surprise, Puppysmiles easily takes control again and actually helps SolOS by activating a broadcast relay, forcing the entity to bide its time and wait for Puppysmiles to be vulnerable.
Creepy Voice is hinted at controlling or influencing the Ghosts that have been following and catching up with Puppysmiles as she moves along Route 52. Creepy Voice manages to convince Puppysmiles to give her control again when they reach the end of Route 52, Creepy offering to take away Puppy's pain. Creepy Voice takes full control of Puppysmiles and demands that the various ponies etc present spread word of her return.
Henrietta and Puppysmiles other friends mock Creepy Voice and how ridiculous she seems to them. Creepy Voice is eventually angered enough to attack them, using the last surviving ghost minion, Sidekick to attack as well as tendrils of blue mist and magic. Henrietta manages to call out to Puppysmiles, finally telling her to let go and move on, showing her how she can reach her mother, only by dying.
Puppysmiles soul is freed from the Mark VI suit binding her to Equestria. Creepy Voice wails and lashes out as the suit begins deflating, eventually killing her as she can no longer hold her shape. The town near her death, Emerald Shores is later renamed to Nightmare's Fall.
Creepy Voice has no body of her own, when she possesses Puppysmiles she turns the fillies gaseous mane into a blue and star speckled one, like Nightmare Moon's. When she takes full control of Puppysmiles body, the gas inside the suit turns blue.
Creepy Voice is a very patient and conniving entity, trying to manipulate Puppysmiles and steal her body. She is also very egotistical and hates being insulted.
Creepy Voice is a powerful magical entity, capable of possessing willing hosts. She can generate tendrils of blue pink cloud from Puppysmiles body and lash out and control several at once. She can also lash out with magical attacks. Without a host though, she cannot materialize or affect the world.