The Battle for Middle-earth II: Passion is a modification of The Battle for Middle-earth II: Passion for PC.
You can now train 6 Black Riders in a horde a the level 3 Siege Works, exactly like in ROTWK, however since there are 6, you can only have 3 Ringwraiths as heroes (Witch-King, Fellbeast and toggle Wraith), which will be shown in the next image.