"Mm... Both." The Ring-Spirit Considers his options... and decides finally to be somewhat straightforward.

"Originally, they were real spirits. But then a long time after most people forgot how to really call on them, someone made a card game based off the original idea. It's technically the card game versions that are called 'duel monsters' as that's what most people are familiar with these days... But originally, the spirits were called 'ka', born from within people's own souls."

A simple explanation! That he can't resist adding a bit of (biased) context to: "And then the pharaoh's priests would harvest them off people for use in duels, because of course they did."