Most fantasy magic is a resource that renews. This one does not. When the gods dwelt in the world, their nearness was magic, as ordinary and inexhaustible as daylight. When they withdrew, they left that nearness behind them like warmth in a room after a fire is out, and every year there is a little less of it. To work magic is to spend a finite, dwindling inheritance.
Strong where they last stood
The remnant is not spread evenly. It pools thickest where the gods lingered longest and thins toward the edges of the world. The Hallowed South, where the Faithful tend the last great light, still runs warm and golden. The Failing Marches of the north are all but bled dry, which is why the dark rises there first. A map of where magic still works is a map of where the gods once stood.
The Void beneath
Against the fading light stands its opposite, the Void: the primordial nothing that the Fallen tapped, an anti-magic that does not fade because it was never made. Where the light is a warmth running out, the Void is a cold that is always waiting. To draw on it is to trade a dwindling power for an endless one, at the cost of being slowly unmade. The dark-mages and the Deepening wizards make that bargain. It always pays, and it is always a mistake.
Three ways to spend a dying thing
Because the light is finite, every wielder of it is really answering one question: what do you do with a resource you cannot replace? The wizards of the Arcane order have split three ways over the answer, and their schism is the death of magic in miniature.
The Wardens hoard it, spending as little as possible so that some remains for a later age; their caution may simply be prolonging the dark. The Free Art spends it boldly and beautifully, and burns through it fastest. The Deepening refuses the choice and reaches for the Void to replace what is lost, and is unmade for the reaching. It is a doom-loop, and it is itself the death of magic.
The living barometer
You do not need a wizard to know how far the age has fallen; you need only watch the small and living things. The river-spirits keep the remnant moving through the world and keep it from going stagnant. The Muses, the least and most numerous of the Natural Order, kindle mortal art and fall silent as the light dies. When the little folk go quiet and the wild things turn feral, the age is deepening. The world itself is the gauge.
Read on
The wizards who fight over the light are the first of The Six Orders. Its source is The Fall. Its keepers, the river-spirits and Muses, live in The Natural Order.