I would be inclined to make the effects almost certainly all modifiers to morale/activation tests.
I would also be inclined not have spells or anything like that.
Maybe you can have some religious leader who can turn mundane events into blessings from the gods.
“See how the clouds came from nowhere to blot out the sun of the Pharaohs force? It is a sign the gods are angry with him, on to victory and smite him whilst he is in disfavour with Ra!” +1 to morale or charge bonuses or something.
The problem with say some seer reading the entrails before, is that if the seer gives bad news (the kind that would give -1 to morale tests) then the king would possibly have the seer killed, so the seer will lie. Or the king will delay the battle until the entrails favour him, so no game, go home, thanks for turning up peeps?
Anyway, I will be watching to see what you do!