The Cornett's Call

 is a side quest in.

Synopsis
Dereo the Lean, a crime lord in the Gullet, seeks a valuable artifact known as the Cornett of Waves. However, it's only a part of a bigger effort to find one of Neketaka's long-lost treasures.

Overview

 * 1) Talk to Dereo the Lean in the Narrows, or be visited by the Cloaked Messenger (after completing Harsh Medicine) to begin the quest.
 * 2) Acquire the Cornett of Waves from Takano's estate in Serpent's Crown, either by stealing it, paying for it, or with a series of other checks.
 * 3) Return to Dereo, and deal with Ateira and her gang on the way over.
 * 4) Head to the Old City, either:
 * 5) * Via the Undercroft - speaking to Gwenfin to open the door leading to the Old City Ruins using the shell.
 * 6) * Via the lift in the Gullet, leading to the Old City Overlook itself.
 * 7) Optionally: Dive down into the pool in the center of the Old City Overlook (area with the lift) to retrieve the Cornett of Depths.
 * 8) Open the Temple of Ondra (located in the northwestern section of the Old City proper) using the shell, kill or avoid the giant cave grub inside. If you have both shells, you can access the hidden room.
 * 9) Inspect the mosaic on the rear wall of the temple, then return to Dereo to complete the quest.

Cornett of Waves

 * Upon meeting Dereo, you will be asked for a favor: retrieve the Cornett of Waves from Takano at his estate. While talking to him, you can notice him wanting the artifact badly with Perception 16 and can pursue the matter with Insight 6 - though he will not reveal any more.
 * If you have already obtained it, you can also show him the Cornett of Depths to trigger the next phase of the quest. Otherwise, acquiring the Cornett of Waves is simple enough.
 * If you already have the Cornett of Depths and you inspected the mosaic before talking to Dereo, you can immediately complete the quest without getting the other shell, but you will miss out on some loot and experience. Dereo will compliment the Watcher on their wondrous abilities.
 * Head to Takano's estate in Serpent's Crown.
 * Moon godlike - Simply ask Takano for it and he will give it to you with no fuss.
 * Steal the cornett yourself - It is found in a locked chest (difficulty 5) at the end of Takano's estate, next to the staircase. The key can be found in the wardrobe upstairs. Time the theft with the movement of the guard in this area.
 * Play on Takano's materialism - Takano will decline an offer to pay him directly for the shell, saying that he already has everything that he could want. Reading his soul reveals otherwise, and allows you to suggest that he could use the money to spruce the manor up, perhaps add a new window. This will set you back, or with Bluff 3, suggesting that Takano's neighbors boast of bigger houses, and that he should out-do them.
 * Convince Takano that the cornett deserves a better keeper
 * Ask "Why is the Cornett of Waves so special?" to discover a bit of the cornett's history, imparting a bit of your own knowledge upon Takano (with an optional, History 1 + Island aumaua + Deadfire Archipelago, Arcana 5 or Religion 5 check).
 * Then tell him "The Cornett of Waves is no mere trophy. It deserves a keeper who knows its history.".
 * Finally, warn him that "Ondra punishes those who refuse to let go" with Religion 3, otherwise Takano will be reluctant to part with it and you'll have to pay.
 * Bluff 5 - Lie to Takano
 * "I shouldn't say this, but I'm on a mission from the palace. A symbol as important as the cornett would help."
 * Then pay, or pass either a History 7 or Diplomacy 4 check.
 * Threaten Takano's life - Requires at least Intimidate 6 to avoid combat, resulting in Moderate Negative reputation with the Huana. Otherwise the other options will start combat with everyone in the estate.
 * Hire an assassin - Rust, an assassin in Delver's Row can fetch the artifact for you. He'll ask for, or if you want to keep Takano alive and a day for him to do the deed. You can use the wait function to skip the time. Curiously, even if you only pay  for his services, Takano will still be alive and well at his estate after you fetch the cornett from Rust, and a second Cornett of Waves will still exist in the Takano estate chest. This is probably a bug.
 * Cornett of Depths - Show Takano that you have a special shell too - This won't get you anywhere, Aloth and Tekēhu will react to this: Pride.
 * Return to Dereo.
 * If you have the Cornett of Waves in your stash while travelling to or from Serpent's Crown, there's a chance you'll encounter Ateira, and her group of thugs. Ateira is the one Dereo originally hired to retrieve the cornett. She'll want the Cornett of Waves, and will be adamant to fight for it. There's no option to give her the shell without a fight, but you can convince her otherwise with one of the following:
 * Diplomacy 4, Diplomacy 6, then Bluff 3 or reputation Rational 2 - The cornett is just a worthless shell, and Dereo is just using it to frame someone. Picking any of the other checks will cause her to attack.
 * Intimidate 4, then Intimidate 6 - The thugs will make easy pickings. Note that this fails if you have Benevolent 4 or a paladin of The Shieldbearers of St. Elcga, with one the thugs calling you "the softest sap in the Deadfire."
 * Bluff 4, then Bluff 6 - Dereo sent you to tie up some loose ends.
 * With Positive reputation (2) with the Príncipi sen Patrena, the thugs recognize that you're in with the Príncipi, and that it's likely that Dereo would send you to clean up.
 * Having reputation Honest 2 will likewise result in them believing you.
 * Or you could just attack her, and get Minor Positive Reputation with Delver's Row Criminals.
 * Returning to Dereo with the Cornett of Waves will gain you Minor Positive reputation with Delver's Row Criminals, and . If you encountered Ateira, you can tell him about it to gain further Minor reputation. At this point you can also push him further about his true motives regarding the cornett with Intellect 12, but to no avail. You're then instructed to take it to the Undercroft below Delver's Row.

Cornett of Depths (optional)

 * If you wish to access the hidden room in Ondra temple, make sure to detour by the Old City Overlook and grab the Cornett of Depths from the sunken statue near the center of the area.
 * With Perception 13, you can hear the calls of the cornett from the pool.
 * With Survival 6, you'll notice the suspicious bones clustered around the water's edge.
 * This scripted interaction requires a character with at least Constitution 16 to retrieve the conch unharmed - otherwise the character will become Fatigued.
 * After obtaining the conch, you'll be attacked by a bunch of skeletons.

Old City Ruins

 * Find Gwenfin in the Undercroft near the entrance to the ancient Huana temple in the northern part, overlooking the bay. With Athletics 4 you can safely take the skiff shortcut on the main dock. Enter the chamber and inspect the statue. Place the cornett in the statue's hand to open the door. The Temple of Ondra isn't where it should be, so guess who's going to look for it inside? Head deeper into the Old City, north through the ruins. The temple you want is in the northwestern corner of the Old City proper. Note that while Gwenfin says she is a bruiser, her health is low and she will not fight back if enemies are lured to her so low level parties cannot count on using her for aid. She will die quite rapidly but it does not result in quest failure.
 * Either the Cornett of Waves or the Cornett of Depths can open the way to the temple, which is guarded by a giant cave grub. Killing it is a major challenge, but you can always sneak by and accomplish your objective.
 * The giant grub is tough, with lots of health, and has strong but very slow attacks. It also summons other grubs to assist it. The trick to killing it is having a cipher cast Tenuous Grasp on it. The giant grub will be charmed, and start killing the other grubs it summoned, reducing your burden. Similarly, the cipher spell Pain Block can help healing your frontline during the long fight.
 * If you take out the Grub Burrow in the Northernmost corner either before or shortly after the fight begins no grubs will spawn and you only have to deal with the giant grub.
 * If you obtained both conchs, place them inside the mural to open the secret wall. This leads to a hidden room containing the unique large shield Cadhu Scalth in the locked and trapped sarcophagus (difficulty 7) on the left, and the unique pollaxe Wahai Pōraga in the sarcophagus on the right.
 * To complete the quest, inspect the Ukaizo mosaic at the back of the temple. Sneak around the giant cave grub if you didn't kill it.
 * Return to Gwenfin and then to Dereo to collect your rewards.

Collect your rewards

 * While talking to Dereo, you can use Bluff 6 or History 7 to find out that Ukaizo is sought by the Consuaglo mes Casitas of the Príncipi as a way to provide for themselves, as one of the richest and most fertile lands in the Deadfire. Dereo will dodge the question if you ask with Insight 4. Uncovering this information allows you to ask for hush money.
 * Dereo will then reward you with the unique blunderbuss Kitchen Stove. You can push him further and ask for a bonus:
 * Streetwise 4 or Diplomacy 4 - An additional and Minor Positive reputation with Delver's Row Criminals.
 * Uncovering Dereo's interest in Ukaizo - An additional and Minor Positive reputation with Delver's Row Criminals.
 * Before leaving, he'll give you Príncipi Colors - presumably handmade by Dereo himself over the course of the quest, as well as the Triumph of Dereo the Lean, the only triumph not given to you for defeating an enemy ship.

Gallery
Interaction 1= Obtaining the Cornett of Depths: