If you've been poking around the Eternal Rift in Nioh 3, you've probably noticed a locked door on the second floor of the Blacksmith's building. Behind it sit two treasure chests stuffed with high-level equipment. The Blackened Key is the only way to open that door, and getting it requires a surprisingly odd chain of events involving dung, a tragic side quest, and a very enthusiastic NPC named Tatsumaro.
Quick answer: Complete the Putrid Paradise Myth in the Heian Era, then return to the Eternal Rift and give dung balls to Tatsumaro the Dung Lover in the cavern beneath the Blacksmith's shop. After enough offerings, he hands over the Blackened Key.
Complete the Putrid Paradise Myth first
Before Tatsumaro appears in the Eternal Rift, you need to finish his associated side quest. The Putrid Paradise Myth is a Heian Era quest that starts in Hachijo, near the Rajomon: Mibu Avenue Shrine. Head toward the swamp and look for Tatsumaro sitting on one of the rooftops. He's desperate to wash off a terrible stench, so your first task is to guide him to a nearby Hot Spring — just follow the quest marker on your map.
While soaking, Tatsumaro mentions that his house has an incredible bath deeper in the swamp, but yokai have overrun the area. Agree to help him, and he'll fight alongside you as you push through the swamp toward a Lesser Crucible. Defeat the wave of enemies to destroy the Crucible, then continue past it to find a ruined house. The "amazing bath" turns out to be a putrid, filthy mess. Tatsumaro, heartbroken but in denial, climbs in anyway and convinces himself it's paradise — only to die moments later. That grim conclusion completes the Putrid Paradise Myth.
Find Tatsumaro the Dung Lover beneath the Blacksmith
After finishing the quest, travel back to the Eternal Rift and enter the Blacksmith's building. Walk to the corner of the shop and look for a way down to the cellar. If you haven't been here before, you'll need to destroy a rock blocking the path using your Guardian Spirit, then descend via the exposed roots.
At the bottom, you'll find Tatsumaro — now going by the title "the Dung Lover" — lingering in the cavern. He has exactly one interest, and it's not conversation.
Gather and offer dung balls to Tatsumaro
Tatsumaro wants dung. There are two varieties you can collect while exploring the world: regular Dung and Noble Dung Balls. Neither has any other practical use, so there's no reason to hold onto them. Hand over everything you've accumulated.
If you don't have enough yet, the most reliable farming method is to kill Wasps and Gaki Chiefs, both of which drop dung balls. The swamp area in Hachijo where you originally encountered Tatsumaro during the Putrid Paradise Myth is a particularly good spot, since several enemies there drop the item naturally.
Each time you offer dung, Tatsumaro rewards you with Smithing Texts — useful crafting blueprints for the Blacksmith. Keep giving him what he asks for until he finally produces the Blackened Key. The in-game description notes that the key is old, darkened, and carries a pungent smell, which tracks given its previous owner.
Unlock the door above the Blacksmith
With the Blackened Key in your inventory, head back upstairs in the Blacksmith's building and climb to the second floor. The previously locked door now opens, revealing a small room containing two treasure chests. The gear inside scales to your progress — expect equipment roughly in the level 110–122 range if you're tackling this during the mid-to-late game. It's a solid upgrade for players who haven't yet reached the final stretch.
The Blackened Key is one of those rewards that's easy to overlook entirely. The Putrid Paradise Myth is a short, self-contained quest with a darkly comedic ending, and the payoff in the Eternal Rift — Smithing Texts plus two chests of high-level loot — makes it well worth the detour. Just make sure you're picking up every dung ball you find along the way.
