The Mikage Furnace is home to treasure, Fatui and nifty anecdotes. It also has a little secret. While exploring, you might’ve come across a key or two that sticks itself in your inventory/quest items.
There are actually three spread throughout the Furnace and they open a gate at the bottom of the facility otherwise locked.
Here’s a quick look where they are:
All keys are inside a chest, but you’ll have to take a good look to find them.
Chest And Key Locations
The first key is on a roof near the very top of the furnace. It’s a bit hard to notice if you aren’t looking. The easiest way to get to it is to jog from the Teleport Waypoint and glide.
From that high up, you’ll see its shape in the corner by a shining onikabuto beetle.
The second location is above the main smelting tower from the Tatara Tales World Quest. This is where two Kairagi always spawn. Go to the top riding the wooden lifts or descend from the cliffs and run along the ridge.
A very noticeable barrier prevents you from getting in. Use the Electrogranum to pass through. Enter the crack in the wall to get to the chest.
The third location is in a strange place. It’s underneath the spot shown on the map below but high up on a cliff outside the Furnace. The least troublesome way to get it is to glide off the cliff and curve around to where the marker is shown.
There are broken planks and a deck with boxes and ropes. The chest is in the corner.
The Arsenal Gate And Achievement
Following your success in getting the keys, we can now open that gate. It’s at the bottom of the Furnace. How you get there doesn’t matter, but it’s pretty simple to jump from the top of facility and drift down. Afterwards, look for the gate in the cliff wall.
If you don’t see it at first, run around the perimeter and you’ll find it easily.
Use the three keys to unlock the gate. As you go inside, a Ruin Cruiser, Scout and Defender pleasantly greet you and try to obliterate you in the small space. Defeat them to explore the cave and open the Luxurious Chest by the campfire in peace.
After opening the chest, 30 Primogems, 250 Adventure EXP, 30000 Mora and the achievement: Blade of Tatara is yours for the keeping.
You get the blueprint for the Katsuragikiri Nagamasa sword as well, and it’s pretty neat.