What it means...Edit
To first understand what the aura is, you need to understand what Vis is. Vis is the magical energy inherent in all things. Every living thing and every piece of inanimate matter contains some Vis. The rare or more powerful the object, the greater the amount of Vis it contains. When a creature dies or an object is destroyed this Vis is released into the atmosphere where it remains till it is reabsorbed by items and creatures. The Aura is this latent Vis energy that isn’t trapped within an object or creature. If an area has a particularly high Aura rating the Vis tends to naturally siphon into areas with lower vis ratings.
How can it be used?Edit
The Aura can be tapped by certain magical creations and be turned into usable Vis. The amount of Aura in an area is finite however and using too much of it can have fateful consequences: The higher an Aura is, the more living things thrive there. Areas with low aura ratings tend to be lifeless and barren.
Occasionally one can find natural crystals of pure Vis deep underground. These slowly grow over time into larger crystals. They grow much faster in areas where the aura has a positive charge. The crystals themselves have no effect on the local aura, unless the aura in the area becomes extremely low. In that case trace amounts of vis are released from the crystals which may cause the aura to replenish at a slow rate. Vis crystals have a myriad of uses which makes them a sought after resource. Care must be taken not to over-harvest them however, since doing so causes instability in the local aura which might lead to an increase in taint level. Without the crystals the aura also cannot replenish should it drop to low levels.