Phoenixes are sentient - willing allies, rather than beasts to be used. The size of a mature phoenix is comparable to a teratorn. They can do the usual resurrection thing, but a phoenix fresh out of the egg would be very small and vulnerable: as vulnerable as a campfire - you can stomp it out, you can pour a bucket of water over it, etc., killing the phoenix permanently. Putting out the fire of a full-grown phoenix by magic is not within the power scope of humans. Growing from baby phoenix to full size takes some time - days, if sufficient fuel is provided, longer if fuel is limited.
Technology available to men is cold weapons (swords, longbows) + limited magic. By limited, I mean a building can be enchanted to be fireproof, but that would be expensive, and the enchantment would need to be maintained - like weatherproofing a house in RL.
One side has about 20 phoenixes, the other side has none. Google says the size of an army would be 7000-15000 soldiers. Letting a phoenix actually die (clarification: I mean, die permanently) is out of the question for the side using them - they are sort of holy. However, that's the side that is going to win the war, and I don't want the presence of the phoenixes to be underwhelming. If possible, I want them to be the gamebreaker that allows victory.
So where can the phoenixes be useful?
I had some thoughts of my own:
- Reconnaissance. Anything the phoenixes can see from the air, they can report back.
- Psychological warfare. Good luck keeping your horses and your peasants from bolting, when a formation of 20 phoenixes (350 square meters of flame) is flying at you. (Estimate taken from teratorn's estimated wing area.)
- Scorched earth. Wheat fields should burn rather well. And phoenixes can burn a lot of them, quite far from the marching army.
All of this, however, feels a little underwhelming. What am I missing? What else can the phoenixes do? How can they be a game-breaker?