After you find Piscilla you can see the huge imposing door.
Open the sliding door, first.
This is the low-spoiler guide for "Huge Imposing Door" puzzle in Gibbous - A Cthulhu Adventure. Unlike in normal walkthrough, you get gradual hints on how to solve the puzzle. Click the big button below to see your first hint. If the hint contains a spoiler, you will be warned about it before clicking.