Use the stairs  in the secret chamber to reach higher level. Position yourself next to the valve  and wait for Willow Blue to come in.
The woman will light a candle  and the ritual will start. Wait until she starts gathering power . Then use the valve  to snuff the candle off. This should trigger a cutscene that ends the task.
Finally the woman will take the mask off and walk away. The woman from the tutorial will appear instead. Willow's mask will enable you to see ghosts near the skulls with candles and more things. Its powers will prove to be very useful.
Once it's over, the day will restart and you will have to deal with another death.