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When there's a new sub-area in Genshin Impact, you'll know there will be a new quest chain for us to discover the lore of said new area. In version 4.6, we have the Petrichorror Dream (or Canticles of Harmony in Fontaine Reputation) to discover what happens to the small island of Fontaine and unlock the new underwater area.
How to Start Petrichorror Dream Quest in Genshin Impact
You will start the quest automatically after walking a little bit in the Nostoi Region (you should have the waypoint unlocked if you already completed the latest Archon Quest). There's not much puzzle in this quest so you shouldn't have much trouble completing this, but I'll still cover as much as I can.
The Shadow Over Petrichor
Once you start the quest (appears on the quest in progress), go to the middle of the village where you can see a bell tower (near the waypoint) to find the right NPC to talk to. There's a lot of dialogue here so get your fingers ready to click.
Continue following the navigation and you will find the talking cat 'Osse.' Osse will be the one unlocking the underwater part of Nostoi Region (called Faded Castle). This is also where you can find the new boss with Arlecchino drop materials.
There will be a new puzzle tutorial called "Symphony" so get use to it because you'll be seeing them a lot more in the new area.
Underwater Nocturne
Follow the quest navigation until you reach the first puzzle of this quest. The Symphony puzzle. The mission is to take the "Blank Score."
What you need to do here is match the slider on automatic reed pipes to the icon on the wall (hold E to play score and press E to pause when the slider is in the right position). After the puzzle is completed, you can extract the Blank Score from the monument.
Keep following the quest navigation until you reach a large stage with a mural on the wall (there's also Golem all over this place). Simply put the Blank Score into the monument to continue the quest. Defeat the golems afterward.
The Last Day of Remuria
This is the sub-quest that will unlock the underwater area (called the Sea of Bygone Eras). The whole map will be dark at first and you need to progress through the quest to light up the map.
When you need to break the chains that bind Scylla, there are 3 spots you need to go and complete each puzzle. The quest will not mark the spots but it's pretty easy to find; you just need to look over Scylla and follow the blue-purple-red-ish chains.
For the first one (south), it will be on the dry land and you'll need to defeat some golems. After that, you can interact with the stone pillar. Get near the pillar and hold E (not your Elemental Skill, but the Symphony power) to release the chain from the stone pillar.
For the second one (east), you need to use Symphony power to stop Washer Octopuses to be in the middle of all Resonant Anemones so the flowers will enter the Resonance state.
(1) Let the Octopuses keep swimming until you're sure they're in the middle of their group of flowers
(2) Tap E to stop them in their tracks. If they're in the right spot, the Resonant Anemones will shine
(3) If the Octopuses are not on the right spot, tap E again so they can move again and repeat (1)
If you're sure they are in the right position but the puzzle isn't completed, try teleporting away from this location and come again. Or better yet, relog the game. If all is good, break the chain by holding E to release the chain from the stone pillar.
For the last one (north), you will meet another group of golems, but you don't have to battle them if you choose the right options to increase their trust, which are all the lower options.
Finally, you're done! Go back to Scylla and he will help you unlock the seal to the next area of the Sea of Bygone Eras. Do as the game told you to do (press some button so Scylla can help you swim) and get into the next area.
Gradus ad Capitolium
Continue following the navigation until you meet Boethius and need to fight Golems with Traveler. Obviously, you can't win the fight so just hold on until Osse (or now called Cassiodor) appears.
Golden Aqueduct Reconstruction: Part 1
For the last puzzle of this quest, you'll need to search for the scattered scores. First of all, operate the lighting mechanism and adjust the camera angle so the light illuminates the horn (Left > Left > Left > Down > Down).
Then, you need to move the birds (just approach them) so they will be placed on top of rocks similar to the one in the mural. Just swim near them and they will fly (even though we're in the water right now) towards their respective rock. If you missed any birds, use Elemental Sight to spot the remaining birds.
Golden Aqueduct Reconstruction: Part 2
Follow the golden bee and it will lead you to another location with the horn and mural. Same as before, (1) operate with the light: Right > Right > Right > Up, and (2) chase after all birds to make sure they return to their places. Use the Humongous Hat Jellyfish so it can launch your characters in the air and pop some Slimes so the birds can move.
Fortune Plango Vulnera
Keep following the quest navigation until you reach the inside of the golden palace (Domus Aurea). Fight Boethius and win the battle with Cassiodor's help.
Play the musical instrument and you will be teleported back to Petrichor. The end... for this quest. There will be a short follow-up quest after this called For Yesterday and Tomorrow.
How to Start For Yesterday and Tomorrow Quest
As mentioned before, this quest is only available after completing the main quest chain in version 4.6. Get close to Este in Petrichor and you will start the quest (no need to wait for daily reset, can be done right after you complete the main quest). The quest is really easy and you can just follow the navigation to complete the quest.