Here’s another lore dump.

Essence fever or Essence sickness:
Since using Essence is literally using your life force and soul to do whatever, using it too much can lead to Essence sickness, resulting in cracks forming on your Essence itself, varying in size depending on the amount of strain.

Can be caused by:
Using Essence too quickly
Using too much Essence in a short burst
Damage from another Essence wielder
Prolonged intense usage with no break

Symptoms:
Headaches
Nausea
Negative change in mood, usually depression but can be different
Weakness in legs/arms
Intense coughing
DEATH! (If severe enough)