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In general, the very name of the skill, "sworn enemy," feels a bit forced. What kind of enemy is it that you can choose? Is it like, "Okay, guys, the lizards ran through and trampled our lawn today. Now they're our sworn enemies!" Something like that?
It's more like training soldiers, explaining the weaknesses and vulnerable points of specific opponents, and also customizing weapons for specific enemies (like, against hydras, use one type of sword; against gargoyles, use another; against golems, use one type of arrow; against zombies, use another).
And "Light Ritual" could be something like this: the hero casts a "virtual training ground" where there's a two-week boost to a specific unit (chosen through dialogue windows), and a battle is started. After the battle, the hero performs a scripted action, entering the Castle, where he can designate that enemy as a "favorite target" (by the way, in the English version, it's exactly "favorite," meaning "liked").
You could spend, say, 1/3 of a turn and some mana on this. The turn and mana costs could depend on the level of the racial skill.
Rain of Arrows is just a spell. You can safely remove it as a perk and replace it with, say, Forest Fury.
P.S. I also often miss Heroes 3.0, where the AI, although it messed up with invisibility (it didn't try to find invisible units), and the ballista could be destroyed with Support, but you could apply Mines, Hive, Crystal, and Wall to Rain of Arrows.