Miyat is a world in the Myriad, one of the longest-inhabited ones. It has about 4,000 years of history from its first contact to the present.
The current year is 117 AD.
Bvatin, also known as door steel, is a rare metal that can open portals when energy is transferred through it. This is done by rubbing or dragging it against another yet undetermined material that generates power, like static electricity. This then works through the blade and opens a portal to the nearest nexus. This other material is typically placed either on the exterior of a scabbard, or on a metal vambrace on the forearm, but its placement can vary and doesn’t matter all that much. Some people even coat the inside of their scabbards so the sword can open as soon as it is withdrawn.
The portal takes about five minutes to close, with your average, 100% bvatin, shortsword-sized blade. These blades are known as keyblades, or maybe key swords or something. After this time it will still be slightly open, but the increasing gravity of the world trying to put itself back together will collapse the portal before it fully closes, as soon as it's small enough that it can’t support itself.
The keyblades will last roughly 50 portal opens before their power is drained. When they’re first made (not sure if it’s forged or cast yet), they have a bright, striking purple (or something else?) color. But as they’re drained it is diluted to a pale white. Thus, ironically, after they’re depleted they become all but worthless and get discarded. It isn’t unusual to see common soldiers bearing white swords into battle—once prized above all, now discarded for the least of warriors.
The last main question is what the amount of metal affects, both in terms of blade size and alloys. Does less metal mean a smaller portal or fewer charges? If the amount of metal doesn’t affect the portal size, then it would be a widespread practice to use as little as possible. What if it affects both? Maybe amount of metal affects both portal size and charges. That makes sense.