Esoteric Texts grant you the ability to unlock and utilise different sets of unique abilities in Sekiro: Shadows Die Twice. And unfortunately, the Ashina Esoteric Text isn’t exactly handed to you.
Shortly before the Ashina Castle Sculptor Idol, you’ll face off against a challenging enemy on horseback in an open field. By defeating Gyoubu Oniwa, you’ll have access to two great things – unlocking Sekiro‘s Prosthetic Arm upgrades, and being able to explore the rest of the area, which ties into the Ashina Esoteric Text. Once the boss is dealt with, head over to the ‘Ashina Castle Gate’ Sculptor Idol to secure your new waypoint, then turn around and head across the field, up the giant staircase. Once at the top, make a sharp right and head into that building. You’ll see a man standing over a defeated enemy. Speak with him, and he’ll talk to you about eradicating the “rats” that have begun to infest Ashina. He’ll ask you if you wish to join him in that adventure. Say yes.
After that, head back across the field and through the door past the Castle Gate idol. Head up the path, take care of the guard there, and grapple up to the high ground near him. Pan your camera to the right, back towards the building where you spoke to the Ashina warrior, and you’ll see a large field with three little enemies there. Two huddled around an item, and one hiding behind a tree, a bit of ways behind them. Their hats are heavily shielded, and thus take an enormous amount of posture damage. Your best bet here is to use the Loaded Axe prosthetic, and smash through their hats. This will open them up for a Deathblow. Wipe out all three, and head back to the Ashina warrior who gave you this mission.
Reveal to him the success of your mission, and he’ll grant you with the Ashina Esoteric Text, which will unlock a new skill tree for you when resting at any Sculptor Idol. It’s also worth noting that outside the building with the Ashina warrior, on the left (when exiting the building) and around the corner you’ll find a merchant. This merchant has a Gourd Seed and a Dragon’s Blood Droplet, as well as Robert’s Firecrackers if you missed those early for your Shinobi Firecracker Prosthetic Tool. This is a great merchant to keep in mind if you’ve been wondering where to find Gourd Seeds or where to find Dragon’s Blood Droplets in Sekiro.