Weg

Weg are black horns that appear on humans who use Forbidden Magic.

Overview
These black horns sprout from the heads of humans who use magic or power from the underworld. They are considered a curse since they indicate a loss in humanity.

A mage who receives power from a devil will sprout horns only while they actively use a significant portion of the devil's power. Devil-possessed mages, like the Dark Triad, sprout horns that resemble their devils' horns, whereas the Dark Disciples, who receive power indirectly from devils, sprout simpler horns. The horns dissipate as soon as the mage stops drawing on the power.

Overuse of Forbidden Magic results in permanent horns.

Trivia

 * Weg is German and Dutch for "way, path" or "gone, missing, vanished".