"A simple hair pin."
The Hair Pin is an item found in Prisoners of the Elder One. It is located in the bedroom on the vanity, and is used to lock one of two doors in the Doll Room in order to trap Vicky. Since the Pretty Slave Ending requires Erik to interact with the vanity a total of five times, the Hair Pin is technically required for that ending as well.
Less usefully, the hair pin can also be used on various other doors around the mansion to accidentally lock them, which may hinder Erik's process if not make it downright impossible to proceed.