A couple ideas:
-Dark Religion - unlocked by the Black Tower of Azkâhar - Once activated, the Mu'adhin (muezzin), standing in a balcony of the Black Tower, convenes all in worship of their King's cult. For x seconds, no buildings can be constructed and no units or heroes can be recruited. After this time, all of the faction’s units gain increased speed and damage for x seconds. (I'm trying to think of a more original final buff.)
Golden King's Vizier - recruited from the Pavilion of the Golden King. Cannot attack, but is surrounded by two guards.
Level 1: Golden King’s Vizier – Allows the Vizier to grow in power (Same as Röac and the Handmaidens).
Level 1: Overseer – Fearful of his wrath, all work with great effort in the Vizier’s presence. Activate to increase by 15% the recruitment speed of nearby military buildings for 30 seconds.
Level 3: Arbiter of Faith – A battalion is chosen to be put under the Vizier’s judgement. If they pass the test, they’re recruited to the King’s personal guard, receiving 30% damage. By failing they will be banished and turn rogue. (50% chance).
Level 5: Enforcer – The Vizier’s enforcers order a caravan to provide the King’s troops with equipment. Reduces the cost of upgrades bought near the chosen caravan for 3 minutes, but it loses 50% speed and armor during said time.
Oakenshield's ideas for abilites (perhaps the top one could even work with a whole new unit):
Slave Driver - target enemy units are marked as potential slaves. When they die, their killer will gain resources equivalent to the cost of the unit.
Sacrifice - target enemy units are marked as enemies of Melkor and will be sacrificed. When they die, nearby allied heroes will be temporarily buffed while nearby enemy heroes will be weakened.