The story
Fiction and adapted settings written by Akitsora Gaming for this title, including invented dialogue and events. This is not a historical or folklore reference. The rules and specification tables define the gameplay and math.
An oban is a block of gold, a furnace of fire, and a whole night of the mint craftsman's hammer.
I. Melting
The gold goes into a crucible and into the furnace; one pull of the bellows and sparks leap from the mouth while the gold slowly turns into a glowing pool.
The craftsman says you can tell how evenly it has melted from the ring of light at the crucible's lip.
II. The mould
The molten gold is poured into black-iron koban moulds and cools into oval blanks; then the hammer goes to work, row after row, until the fine hammered grain shows.
The number on a gold nugget rolling off the casting bench is the weight of the spin (base game ×2 to ×20).
III. Opening the furnace
Gather 4 molten gold and the furnace door swings open, the gold running mould after mould. The next 15 spins play out in the night vault, the multiplier growing all the way (×2 to ×500).
3 more molten gold add 5 more spins.
IV. The hallmark
Last of all the hallmark hammer comes down on the oban — with that mark, it is gold of the mint.