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These thrall units are extremely weak, but can serve as the first line of defense against a variety of enemies. The cost of creating a Thrall Master is equal to that of the lowest of the four unit types it can summon. It costs nothing for a Thrall Master to summon a unit, but it can only summon one type of unit at a time. Thrall Masters and their thralls cannot be upgraded with other armor or weapons. Additionally, if the Thrall Master is killed in action, the entire battalion dies as well. This is why the Thrall Master always remains in the back of the battalion for protection.
The Thrall Master is one unit in the game: The Lord of the Rings: The Battle for Middle-earth II - The Rise of the Witch-king.