What is Aura?
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?
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.
You can suck the taint out of an area by using a condenser with the researched upgrade Concentrated Evil. Then stick a tainted vis crystal inside to power it and you now have a taint syphon. Just a warning, this thing will generate ONLY taint, so you can sequester it in tanks (that hopefully will never break) Or feed it into a series of purifiers to kill it.
If you want to increase the aura, you can either throw Concentrated Vis potions around and/or place Totems of Dawn. These must be researched before they can be constructed and will not only raise the levels of Vis but will also prevent the growth and spread of Taint in the chunk in which it is placed.
Silverwood Trees are also important as they perform the same function.
Vis Crystals do NOT increase the local aura at all but instead require a positive charge in order to grow. They will, however, keep the levels of vis up when they get dangerously low.