Traditionally, a popular way to show that a character is evil is the use of colored contact lenses in live action, or colored or glowing eyes in animation.
Since the 1990s, however, a new trend has emerged - the Black Eyes of Evil.
So we have black sclera contacts now.
These days, when someone is possessed by demons, dabbling in the dark arts, psychically dominated by evil aliens, or gone evil in some other way, their eyes will go perfectly black with no whites at all.
To have this effect, we have to use Black Sclera contacts
to cover your entire eyes.