I don't know if this is a bug or not (you never really know with this game), maybe someone around here will have an anwser !
So I ended up winning this battle (I play Pangaea) :
My objective was to neutralize Niefel's SC with a combination of Charm/Enslave Mind. They had low MR and I had high Penetration.
It worked for one of them. I saw it during the battle : I think it was an Enslave Mind (not sure), and one of the Niefel Jarls turned against his ancient brothers, two turns before they fled.
The problem is, after the battle I can see no Niefel Jarl in my army ! I'm sure the giant didn't die during the battle, I saw him standing next to my tarrasques, taking plenty of poison damage right after the last turn of battle, and surviving.
So no giant, but instead, I got this :
How the hell did this Great Eagle appear and join my army ?
Now look closer at it : it has experience, vulnerability to fire and cold power... So there's no doubt about it : the Niefel Jarl converted into a Great Eagle...
Can anyone explain this ?