Board Thread:Suggestions forum/@comment-31382381-20170225024029/@comment-31382381-20170228122124

Naturally, their worship of the dark, plus the Mordor Black Numenorean's staying relatively pure would have strengthened them, as well as their continual contact with the sorcery of Sauron.

And remember, Swan Knights are not purebreds. Yes, relatively pure, even with maybe some Elvish blood, but even they have dwindled. And they don't have the sorcery of Sauron.