One great way to give a humanoid character a more demonic and intimidating look is to make the sclera of their eyes jet black. This works not only because it’s the natural opposite of the norm, but also because it allows whatever color you choose for the iris to stand out that much more. Black scleras will leave your audience wondering just what kind of soul lies behind those glowing eyes.
Diablo (from That Time I Got Reincarnated As A Slime) certainly has a devilish glare that suits his name, don’t you think?
Have any of the eyes below left you hypnotized?