The Black Knight is a V E R Y good F2P (Free to Play) Unit and it's not advisable to fodder him off. He's extremely useful with his statline and with his exclusive skills, Alondite and Black Luna. If you want a good Enemy Phase skill, a Breath skill (Warding Breath or Steady Breath) is more advisable since it's more worth it (ironically) on some units that Steady Stance 3.
Sadly you'll have to kill off a 5* if you want to have the skill on another unit. The only two units that have Steady Stance 3 are Black Knight and Shiro(5* Exclusive).
I personally wouldn't kill him off, but that's because I keep all TT units. Whether or not they're of use is another story.
Keep him. Out of all the existing TT units, he is easily the best, and Steady Stance 3 is not that good of a skill in comparison. Like many others have said before, if you really want some enemy phase buffs, there is other ways to get them, for instance Steady Breath from brave Ike who has a really good chance to appear in Legendary or skill-themed banners, as it's a much desired skill everyone wants. Black Knight on the other hand, you will most likely not get him back.
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