Fulminated Mercury is a rare upgrade material in Sekiro: Shadows Die Twice. It is used to upgrade your Explosive Prosthetic Tool in the late game. If you are not in the late game, read on with care, there will be some spoilers in this guide.
Where To Find Fulminated Mercury
Fulminated Mercury can drop from enemies, specifically the enemy with the large cannon at the Gun Fort. Farming this guy isn’t exactly ideal, however, as the drop is so rare. What you want to do is wait until the late game, when the Ministry begins to attack you at the Ashina Castle Idol. This is the perfect place to farm this, as many of the enemies you face will drop the item.
When you reach this stage in the game, head for the Ashina Castle Idol. Use a Misou Possession Ballon to increase the chance that enemies will cause drops. This method means we need to rest at the Idol to reset enemies repeatedly, but that is fine as the Possession Ballon will still work, even after resting.
Now, you need to use the Golden Vortex, which is an upgrade of the Divine Abduction Tool. You do need to have two Fulminated Mercury to upgrade to the Golden Vortex, but you will need to get those the old fashioned way.
Once your Possession Balloon is active, and you have your Golden Vortex, just run up the stairs to the groups of enemies ahead of you. Use the Golden Vortex, which effectively loots all surrounding enemies, run back down the stairs and reset. Repeat this until you have all the Fulminated Mercury that you need.
The Bodhisattva Valley
When playing the story, you will eventually come to the Bodhisattva Valley, after you fight Genichiro. If you do discover this area, there is a bag of Fulminated Mercury behind a rock, on an island with lots of monkeys on it.
Honestly, the easiest thing to do is wait for the Ministry to show up, get two bags from the old fashioned way, then upgrade to the Golden Vortex and use that method to loot as much as you need swiftly!
That should get you all the Fulminated Mercury that you need to upgrade your Shinobi Tools!