The red continent is an ice and snow covered region far to the south. Yetis prowl the frozen wastes and devils live just below the ground in a network of boiling hot tunnels.

I envision the devils as white-furred, with cloven feet and ram-like horns.

I like the red flavour so far.

Brainstorming: I wonder how they react to the Aer. Do they have enough civilisation to understand the mana being stolen, or are they just defending their mountains? Are they tricksters or berserkers (the all-in mechanic hints towards berserkers). Are they equally hostile to any of the other mono-coloured races that appear? Are they passionate about things other than defending their territory?

My initial reaction would be "Nah. They're cool. They just rage against everything, and we don't need to think further than that."

Besides causing a problem when writing out the flavor text for 40 or so cards, though, I think the plan is that all the planes are starting to push back on the aerans. If that's the case, they've got to be cognizant of their 'rulers', even on a vague conceptual basis. And, knowing red, they would do everything within their power to escape the cage they find themselves in.

Other question: Who do you think are more intelligent? The Snow devils, or the yetis?