I am uncertain whether these could be considered universal, but generally illusionists break them down into four parts: length, strength, potency, and power. I feed from the power of a soul, which is essentially the stamina a creature has at any given time. When power grows low, eating or sleeping replenishes it.

[She pauses for a moment and her voice grows softer.] Normally illusionists feed from the potency of a soul. It defines everything that makes a person who they are, including their emotions. To remove it is dangerous, often painful, and... it changes them.