What's the deal with lizardmen?
What if they don't like the cold?
What if OGRES don't like the cold? It makes sense for a race from a volcanic rainforesty region to love fire magic, and then naturally the rebellion can be all about the ice magic. That's a fun reversal- I feel like Ice isn't usually a good thing. But ice keeps your food from rotting, keeps the ogres away. Plus you can make water out of it pretty easily, so ice is really just swole water.
So lizardmen can't deal with the cold because they're from the warm, swampy regions on the border with Zahar. They were the first conquered by the ogres, and fought them the longest and the hardest, and they're among the most dedicated members of the Boryan empire. Unfortunately, Caesar started the rebellion in the far north, and the cultural history of the war is one of ice versus fire. The capital city, and indeed, most important cities, are in areas with brutal winters as part of this cultural/military legacy. Lizardmen who can't take the cold can't be where the important things are happening! Lizardmen who can't take the cold are seen as unpatriotic. Lizardmen who can't take the cold guard the border with Zahar, and sacrifice themselves by the thousand every time there's a big resurgence of evil. They're the Night's Watch, and get about as much respect and gratitude in setting (That is to say, none.)
Also, ice and chains is super metal as a motif for an empire.