The Tenebris Somnia moonlight is a window, and reaching it needs the Red Pentacle Stone, which drops when you defeat Fumus.

The Tenebris Somnia moonlight location and how to reach it

Step 1 — the Floor 3 locked bathroom.

  1. Go back to Floor 3 and into Iván's apartment.
  2. Use the Red Pentacle Stone on the locked bathroom door to open it.
  3. While you are in there, take the Chemicals from the table, because you need them later for photo development.

Step 2 — the red-marked door in the production-company lobby.

  1. Find the white door marked with a red symbol and use the Red Pentacle Stone on it.
  2. The stone crumbles after this second use, so you only get two doors out of it.

Step 3 — the vacation room.

  1. Go through the hotel room and sit through the dialogue.
  2. Approach the window where Julia's statue is standing and use the Bottle With Three Doses of Water on it.
  3. The window exposes the room to the moonlight and the bottle becomes the Purified Water.

The Fumus fight is where the Red Pentacle Stone comes from, and it is the only source — the stone is consumed twice and there is no second one. The Fumus fight itself is in the boss guide.

What the Purified Water opens

  1. Take the Purified Water back to the Cursed Film Canister in the Film History classroom on the first floor of the film school.
  2. Use it on the canister before you try to pick it up.
  3. Cleansing it gives you the third Film Canister.
  4. Take that canister to the projector room to open the last theater box and get the Cold Chisel.

The same moonlight theme returns at the very end of the game: the Viridis Knife is used at the sealed hotel door north of the production-company lobby to expose it to moonlight, and that is what forges the Aurifer. Both uses are late enough that the route around them is planned from the start. Where the cleansed canister sits in the overall route is in the walkthrough.