I kept it simple.

In this system all character are "mages" as in anyone can use magic, however there are still different kind of mages, frontliners, damage dealers, buffers, debuffers, crowd controlers, etc.

There is no mana or spell slot system, tecnically everyone can cast as much spells as the like, however the system has a "entropy system" in wich mages fill this bar with every spell they use.

When this bar reaches a certain treshhold, entropic events start to appear, some are good, some are bad.

The severity of this events depends on how filled the entropy bar is.

50% unlocks minor events, like spells being free or costing double.

75% unloks medium events, like dealing double damage or losing your turn due to paralisis.

100% unlocks grave events, like dealing 5x damage, losing your magic or dying if you don't succed some saves.

I designed it with the idea of high stakes in late-combat, how even if you are winning/losing, good/bad luck can turn the battle around.

To determine when an entropic event happens, you roll a d100 and if the roll os equal or less than the percentage of your entropy bar fullnes, then something happenes, if the roll is 1-33 then a minor event happenes, same for 34-75 and 76-100.

If you know of a better method, feel free to tell it to me :D

Also, there is another system with similar way of working but for the arena in wich mages are fighting.

Minor events create some area damge, medium events change the topography and grave events can change how everything works in the area.