O_o ? I don't recall saying that you NEED to take a turn to prepare a spell. From what you had said in your initial post, it sounded like you were preparing the spell but did not intend on using it this turn.
"In the end, he decides to brace himself, ready to cast a protection spell on the boy if required... "
I needed clarification on this because this may greatly impact what happens here.
The only real restriction on spells is you can't cast more than one spell in a turn. If you say you're using the protection spell, basically, your spell and Rethagos's spell would go off within seconds of each other when I resolve the action. Your spells always go off on the same turn you cast them, though that doesn't always mean they will work effectively.
