Light or Dark magic.

Well, you could just as easily take an Elven Winghael, for example; he just has low strength, as most of his strength went into knowledge.

It's easier to take Retribution for the Knight, and yes, the Elf does spend a lot of stats on knowledge. Haven has a hero who is probably better than Winghael overall, hmm, Miev.
I forgot about mini-arts; you can create them with a Mage and transfer troops with mini-arts to the Knight.