I strongly advise you against using the Black Knights, at least until you free Moro. Every fight you have them in your army (including reserve), the power of darkness will rise by 1% per battle (the counter can go over 100% without limit). In the end, you will be able to free him, but the sooner you can do it, the better.
Unless you set yourself a goal to play with 1 unit only throughout the game, I strongly suggest you get some dragons to do the fighting till you free Moro.
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