The strange girl is a girl north-east of the Prifddinas lodestone that is very excited about Hallowe'en. Talking to her on 31 October and agreeing to try on a costume dressed the player in what appeared to be an ectoplasm-covered bedsheet, visually matching the one used in the Ghost Ahoy quest.
Whilst wearing the costume, players were unable to run, rest or access most interfaces. The outfit was also not shown as an equipped item. Attempting to interact with NPCs, scenery, items etc. caused the costume to disappear with the chatbox message: You no longer feel so spooky.. Curiously, talking to the girl again also triggered this, rather than any additional dialogue. However logging out or going to the lobby would not cause the outfit to disappear.
Before the 31 October, players were asked to return on that date to take part in the costume gathering, and after 31 October the girl just reminisces about the event. She also has a male counterpart in Burthorpe.