Im guessing the battle started, the enemy had fly cast on them (useful for bringing small parts of armies towards you at a time, not so useful 1v1) and then flew over and killed your assasin.
No. It wasnt my assassin. If you will check my save you will see that my commander got flight from the assassin after first few casts have missed. And the assassin already had flight.
My theory is that the assassin was trying to cast some other offensive spell, but there is a bug in spell numbers.