Fervent believers who refused to renounce their religion at the risk of death were called martyrs. Witnessing the death of a martyr had a powerful effect on anyone of weak faith. The cause of Christianity, for example, was strengthened by the spectacle of believers who willingly chose death over conversion to a pagan religion then accepted by the Romans.
Benefit: Lets you instantly convert an enemy unit by sacrificing one of your Priests. You cannot use Martyrdom to convert an enemy Priest.