very basic idea of it for A Silvertongue's Tale
everyone is capable of magic, some more so than others.
Magic shows with different eye colors or even hair colors (like for instance, Kerren’s pink eyes). This is the same for both Mythicals and humans. Magic drains the normal user of their endurance. Think of running a marathon. If they push past that limit, however, their magical energy will start to feed off their life force, killing them if they are not careful. As a result of the excess energy as well, those who have an average and up abilitiy of magic, need to eat more to help with the energy loss from their magic (think dragon ball z)
Some people have an extremely minimal amount of magic, one character (who has yet to be introduced), she has an extremely minimal amount of magic, but it's enough that allows her to use spell cards (like from the game Kuon). The basic ones can help with defense, such as throwing one can turn it into a fireball going to the enemy, or an ice spike, and they use minimal magic from her. But the ones that could make her pass out ,as they sap the energy/endurance from her would be the summoning cards.
so, the average person still has these limitations.
The only ones who are technically 'free' from the limitations of it taking their lifespan are Deaths and Gaias, as they are made immortal by the High council. Though they do have rules about how they can use their magic. They must follow the way it's written, so to speak. But they can still get drained. Normally, they have one day where they sleep all day, kinda like a mini hibernation (and that reminds me of another post to make later about the Deaths and Gaias), but if they overuse their magic, they'll have more than the one sleep day per month.
There's another type of magic, however, that even the Gaias and Deaths won't use. and that is blood magic. As guessed by the name, the magic is powered by your blood. It can give the user extremely powerful magic, however, it can kill the user extremely quickly as well. For most everyone, it's considered taboo, though even powerful mages keep blood magic incantations in the back of their minds as a last resort.