Rams Horns

The use of Ram's Horns should be pretty popular if you have any characters that are Infernal, this goes doubles for demonic characters and members of the Order of the Fallen. The imagery of horns and their association goes back to medieval times when pre-Christian pagan gods and mythological creatures were slowly changed on a cultural level to represent sin, the dark side of human nature, and the conceptualization of evil itself. 
Since then, straight-up demonic characters are frequently pictured with horns on their heads, particularly those of a ram. 
Many real-life artists make use of ram's horns for their darker aesthetic and shock-value, such as Katty Perry during one of her Oscar performances.