Every creature has an Anima. What an anima is exactly is up for debate, but it can often be thought of as a creature’s “aura” or “soul”. What this means in game terms is when certain magical spells, items, or effects happen, they can trigger your Anima.
At its most basic form, an anima is a colored glow that surrounds your body. However, as a person grows in power, their anima can change. Certain traumatic or special events can change a person’s anima.
An anima can be many things. For instance, a necromancer may have spectral skulls floating around him, a druid may have spectral flowers bloom, a Tiefling may have red fire burn around him. An anima has a color, but is transparent. An anima has no mechanical advantages in game terms. An anima cannot provide concealment or affect the world in any way.
In game terms, an anima is just an element for added roleplay and description. Generally, you cannot will your anima to manifest, only certain things can trigger it. For example, a Cleric’s Channel Energy class feature or a Barbarian’s rage power trigger a character’s anima. Because of this, martial classes like the fighter may not see it appear very often. However, magical items can also manifest an anima. If a magical item glows, it is almost always the color of the person’s anima.
There are no specific rules on what can or cannot invoke an anima, or what it can look like. This is for the GM and players to decide on their own.
Note: The idea for an Anima has come from a few sources, but most importantly the webcomic “Goblins”. The link is here, if you would like to find examples of anima: http://www.goblinscomic.com/