I realized that I’ve explained a lot about how the magic of Windemere works, but I never really went into details with how it can stop working. There are dead pockets (no magic can work) and cataclysmic regions (magic doesn’t work correctly), which alter spells. Those are all outward influences, but there is an internal issue that can utterly destroy a caster: TRAUMA.
Remember that aura in Windemere is life energy, so casters are throwing part of themselves when they cast spells. This leaves them open to negative side-effects that result in trauma. There are various methods that this can happen. As seen with Nyx, extremely powerful spells create a connection to targets and you can feel the death of that target if it’s a combat spell. This caused her to lose access to her magic for a time, which was combined with her losing faith in her own abilities. A caster can also be struck by backlash if they cast a spell that they don’t fully understand or are too inexperienced to attempt. Finally, there are a few counterspells that are designed to cause trauma, but they also run the risk of affecting their caster as well.
The affects of trauma on an aura can range from inability to cast magic to simply being reduced in power. A more powerful caster will suffer a greater affect even though they are less likely to be hit with debilitating trauma. In other words, Trinity will get hit harder than Sari even though Sari has a higher chance of being traumatized. Though I will admit that Sari is rather difficult to damage to that extent due to her positive nature and already surviving several hardships. Anyway, there are several factors that relate to the level of damage. The degree can be different for every caster and even every time. Most times it involves a mental block caused by a subconscious fear of being hurt again, so therapy has been known to help.
Now that isn’t to say aura trauma can always be undone. Aedyn mentions at one point that priests do have spells to revive magic, but there are times where the caster is simply done. Their aura has been severed and lost to the ether, which means they can never cast a spell again. Since all living things have an aura, these people have just enough to avoid becoming undead. If they lost it all then they would become either a zombie or, if evil-natured, a Lich. So you can see that trauma can create one of the darkest creatures outside of demons. Tales of these incidents is one of the reasons why not everyone tries to become a caster.
So there you have the downside of magic in Windemere.