Mark of Shame

The Mark of Shame is given to those who have either disgraced or turned away from the Covenant, i.e. heresy. The Mark of Shame is imprinted permanently on the body of the person by means of branding.

After being branded with it, the person is executed soon thereafter. The only known exception was the Commander in charge of the protection of Installation 04. He became the Arbiter in order to settle the act of Heresy on Threshold.

Trivia
The Mark is available as a Halo 2 Multiplayer Emblem. Also if you look closely on Elites backs in Halo: Combat Evolved, there is a semi-glowing circle that has the mark of shame.