Amulet Keys are special objects that allow entrance through stabilized portals in and out of the Yokai Realm.
Stabilized PortalsA natural portal will usually only open during certain times of the day, when the most light is hitting them and reflecting off of the rich pools of Yokai Gold underneath. Stabilized portals are natural portals that have been bound to a spell, often a physical sigil in the area, and able to be opened and closed at will.
Mechanics & Inner Workings
The amulets are able to open stable portals by channeling mystic energy into them. The yokai gold that is in both interact with each other and the spell to open the portal regardless of time of day. This process will only work if the amulet is intact, so it it's surface is cracked or damage, it's power will be weaker, and if it's broken, it will stop working entirely.
Amulets appear as round discs made from yokai gold. One one side they will be etched with the same mystic sigil that stabilizes the portal and on the other, there will usually be a customized symbol, usually of the clan it belonged to, or the symbol of the yokai region it was from.
After the Yokai War, natural portals needed to be regulated and guarded by The Guardian Clans. They did this by creating a system of stable portals bound by a spell and able to be opened with the use of the Amulet Keys. These keys were not handed out willy-nilly. Only trusted individuals were allowed to have them. For Yokai this included traders, ambassadors, lords, daimyo's and other forms of power. For humans, only those in the guardian clans who have trained and earned their place in the clan can get one. And just like any right, if it's misused, it can be taken away. Keys were never something people would flash around as having, as they would become targets of thieves who would steal a key in order to get free access to a stable portal. Many yokai are desperate to travel and so owners of them would have to be very careful and guard them with their lives.