Once researched, Priests gain the ability to instantly convert an enemy unit when the Priest is sacrificed (press DELETE). Sacrifice can only be used by the player. However, to some players, it is deemed useless since the cost for it is expensive and so is the Priest.
In campaigns or quests, it can be useful if an ultimate unit crucial for victory is hard to get such as Zenobia's Tower and other powerful heroes. In regular games, it is usually more cost effective to just use more Priests to convert enemy units instead of researching Sacrifice.
"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."—The Rise of Rome manual
|Age of Empires|
|Bronze Age||Astrology · Mysticism · Polytheism|
|Iron Age||Afterlife · Monotheism · Fanaticism · Zealotry|
|The Rise of Rome|
|Iron Age||Medicine · Sacrifice|