Need help! summer themed faction!

If it's a kind of story where sex magic can appropriately be included, that seems to me to fit very well with summer. Heat of passion.

Other possibilities could include thunder/lightning magic (if this is the kind of climate that has summer thunderstorms), and plant magic. Both gardens and wild plants would be flourishing in the summer. If there are particular animals that show up a lot in the summer, they could be tied to the magic. Fun fact: the astrological signs of Cancer and Leo were so named because in Mesopotamia, where they originated, the months when the sun passed through them were months when crabs came ashore in large numbers to molt and mate and when lions migrated, respectively. If your world has any such seasonal markers, they could be animals of the season.

 

Some ideas:
- Pyromancy is the obvious one (so I get why you would look for something else).
- Berserkers; heat and rage seem linked to me. It also would be nice in opposition of your knights of winter.
- Undying/Phoenix; a magic where death is temporary, but might hurt the user in another way.
- Illusion; light and heat are two factors that can create visual illusions (fata morgana, for instance).

 

And you know what the berserkers did: they turned into bears. Literally, if you asked them. (Historians say they were just wearing bear suits, but what do they know?)

A summer bear is the fierce one. That's when it's voraciously eating everything in sight.