Akihara's Blazing Flame

The Akihara clan, specialized in blacksmithing, was a huge part in the World’s Breach, as they were equipping the warriors of Kumiho. After the end of the war, they settled, like many other clans, in the Katokai Bay, and helped built the now capital of Seigashi.
The leader of the clan came across a neverending flame inside a rocky pit, and manage to bring it back to the clan's newly built forge. After years of trying to figure out why the flame never weakened, one of the best blacksmith in the clan tried to forge a simple dagger inside the flame.
The result was most surprising as a clearly magical aura was attached to the weapon and without the help of any magical means in the craftsmanship.
The dagger, still exposed in the hall of the workshop today, has never rusted nor lose it’s sharpness.

Utility

The only use of the flame is to forge weapons or special commissions from the army of Katokai. The Shinsengumi is fully equipped with weapons coming from the Akihara's clan, as the quality was never matched.

Manufacturing

It is basic blacksmithing techniques but a very precise execution is needed. The Akihara clan discovered that the more pure the ores are to craft a weapon, the higher is the quality of it. Simple iron or steel weapon have simpler "enchantments" but the current best blacksmith of the clan, Tsubaki Akihara, has tried to forge with platinum or adamantum and it yield better weapons, and some also gathered the fire aspect of the Blazing Flame.

In game mechanics, the weapons crafted by the Akihara clan follow these rules :
The Akihara clan is specialized in blade type weapons. Making an axe or hammer type weapon will demand more time and money to be created, and are never available in the shop , even for the lowest tier, and have to be commissioned. The clan also do not craft any kind of armor but if you choose to allow it, they will never be magical.
Simple weapons : They are magical weapons +1, +2 or +3, depending on the time and money your players are willing to pay. The +1 are always available, the +2 demanding 2 to 3 weeks to make, and the +3 being at least a month. The shop can have some +2 or +3 already prepared depending of your game, but shouldn't be common.
Fire weapons : Those weapons are only for commissions, and take a few months to make. The weapons who have the fire properties of the Blazing Flame are considered Flame Tongues weapons, as described in the Dungeon Master's Guide. You can adapt the price for the game you are running but keep in mind that this is a type of rare magic item, and therefore should be at least a few thousands gold to commission.
The Shinsengumi is fully equipped with the weapon of the Akihara clan. The +1 being for the majority of the standard guards, +2 for captains or lieutenants and +3 being for the high grade. The leader of the Shinsengumi however as a Flame Tongue Katana +3 which is considered a long sword.

Access & Availability

Only the Akihara clan has access to the never-ending flame, now known as the Blazing Flame, as it is a unique artefact, as far as the knowledge goes.
But the simpler weapons are for sell for those who can afford them, and the contract with the Empire to equip the Shinsengumi made the weapons known to the public.
The clan can fulfill commissions from adventurers or Noble Houses, but they must have an authorization from the Seigashi Council.

Complexity

Nobody really knows why this flame can't be extinguished or where it came from, and the Akihara clan doesn't allows the Yamata Academy near it for research as they don't want them to take advantage of it, or simply take it for themselves as part of the "Arcane Control".

Discovery

In a rocky pit, buried deep down the mountain side.