Of course a TCG doesn't really need a story, but mine has one to give you some context about what's going on. Here's the origin of our summoners:
The black goo called “Liquid Shadow” briefly appeared on the dark web a couple of years ago.
Many brushed it off as somebody trying to sell bottled ferrofluid for a pile of bitcoin. But few with expendable income took the plunge and bought it. What they’ve received in the mail changed their life.
It jumped into their skin at the first touch, soaked in and disappeared. They described what happened next as feeling it rush through their veins, spreading throughout their body, then rushing to stop in their head.
With only an empty container, and a page of mysterious instructions, nobody believed them, and it didn’t show in any test that doctors tried, but after a while, it bubbled to the surface of their skin.
It didn’t take them long before they figured out how to focus on it with their mind, and force it out as a black blob they could hold in their hand.
Trying to get rid of it, they threw the goo, and found out it takes the shape of monsters after hitting the ground.
Sounds like an origin story for a super mechanic, with oil stains that he can't get rid of.
The People coming in contact with enough Unbound Liquid Shadow gain the power to produce and control it, they are called shadow slingers or summoners. Each summoner has a cap number that indicates how many Liquid Shadow cards he/she can hold, and each monster has a Liquid Shadow cost to cast.