What's so special about that? A second unit simply appears... You have to fight it first, and then the unit will appear in your guild.

No, it's just that an Elven hero should carry a "booth" with the sworn enemies so that he can switch them at any moment (the city shows who his sworn enemy is, so you can retrain them at the last moment, and that's it - the enemy is out of luck).
By the way, can you retrain units in the Fortress on the hill?

Added 8 minutes later
What's also interesting is that complex neutral units will appear: you can combine griffins and angels.
I wonder if Nival will add outposts that don't disappear after the hero passes by?