Just like the Dark Souls games before it, Sekiro: Shadows Die Twice is a cryptic and punishing experience that forces you to adapt to everything it throws at you. While the story itself is the most straightforward out of any of the recent games FromSoftware has released, there are still plenty of items that retain that same mysterious element as before, almost taunting the player to figure out what they do or how to use them. Truly Precious Bait only shows up twice in the game, but even if you find one there's no indication as to what it will actually do. If you want to uncover the secrets of how to get and use Truly Precious Bait, this guide will be far more useful than any item description.
How To Get Truly Precious Bait
The Truly Precious Bait items are both related to the Pot Noble side quest in Sekiro: Shadows Die Twice, and can be confused with the regular Precious Bait item.
The two places you can get Truly Precious Bait are from the Pot Noble merchants Harunaga and Koremori. They won't have it initially, but as you continue to give them more carp scales they will each reward you with one of the two Truly Precious Bait items. While they may have the same name, there is an important difference between them. One will make reference to the Hirata Estate, and the other to the Fountainhead Palace merchants.
Remember this when deciding which one you want to use.
After you get the Truly Precious Bait from the first Pot Noble merchant in the Hirata Estate, he will give you the vague instructions to "bring it to the Great Carp of the divine realm... Then, I will finally become a carp... and bestow upon you my secret treasure."
Follow the promise of treasure back to the Fountainhead Palace. Go through the building you spawn beside, out the front doors, and take an immediate left toward the waterfall. It looks like a dead end here, but if you jump off the far right side, next to the small tree, a grapple point will appear on the cliff. Yank yourself up into a cave with some lizards crawling along the floor and ceiling. Jump down onto the roof and to the second Pot Merchant. He will talk some trash about the other merchant.
Listen to his secret and take his Truly Precious Bait, with the instructions to feed it to the Great Carp stealthily for yet another secret treasure. Of course, this being a Souls game, you can't use both and it is never clear which, if any, NPC is trustworthy.
How To Use Truly Precious Bait
The location of the Great Carp is in the Feeding Grounds, which has its own Sculptor's Idol. If you haven't found that location yet, teleport to the Sanctuary, go down the stairs and outside to climb up onto the roof. Head to the far left corner and grapple to a branch leading you just before the feeding platform near the water.
You can speak to the giant spirit thing here if you want, but he just tells you to ring the bell and feed the Great Carp. Ring the bell to the right and a massive carp will swim up to the railing. You are given two options here: Use Precious Bait, or Select Truly Precious Bait. Select the second option and you can choose which of the merchants' bait you wish to feed to the Great Carp.
Regardless of which merchant's bait you use, the results are almost identical. The Giant Carp will always die, as well as the opposing merchant of the one whose bait you feed to it. The surviving one will turn into a carp and also take on the inventory of the other, so no matter what you won't lose access to any items. The secret treasure, again, is the same for either option: a Lapis Lazuli.