Back in the hypernarc underground of the 2040s the line between governments and drug cartels became increasingly blurred, with adherents of the cultist Papa Lawrence infiltrating law enforcement groups around the world and cash-strapped agencies needing to pad out their expense accounts a little, it was never really clear whose side anyone was on.

The real power, such as it was, lay with the accounting and tax collection departments of various government and ultragovernmental bodies, so obviously it was only a matter of time before they became corrupted too. Having learned well the lesson of Al Capone, criminal syndicates made it their business to stay one step ahead of the number crunchers by having inside operatives at every level.

Even with that, regular purges and corruption investigations began to take their toll, so the need for a means to move money around in a way that was difficult or impossible to trace became urgent, and so was born the shadow card.

These black credit cards with a faint tracery of holographic neon were the physical elements of a bizarre parallel financial system which was developed and grown in dingy back alleys and rattling conduits of the sprawling net by an unknown and probably insane 'grammer stoked on zip juice.

Part AI, part smart network, it spontaneously generates trillions of connections between nearly infinite numbers of financial vehicles, bank accounts and other economic arcologies, and have a unique one-time key attached to each shadow card. They're almost completely invisible, piggybacking on legitimate financial traffic to learn about new forms of encryption, until they're activated.

It's relatively easy for each card to be associated with one or more bank accounts, but once activated they can't be rerouted to other accounts. They can then be used as normal credit cards but it's almost completely impossible for anyone watching the accounts to trace transfers to their destinations. So low-key operatives can travel freely, with the money they spend apparently vanishing into thin air, among other things.

Even the best program has its limits, of course, so every card has a miniature counter embedded in the surface - when it reaches zero, it's no longer safe to use the cards to withdraw money. These may tick down faster or slower depending on how much attention is being paid to the debit account.

The original creator (and their sponsoring syndicate, if any) are long gone, so shadow cards have become much sought-after unique items in the criminal underworld and espionage circles, especially if they haven't been activated yet. Legend has it that there are some still connected to vast untapped narco banks, forgotten and untouchable, and a whole lot of blood has been spilled to get them.