@SAF I would picture Aine gawking at a shining barrier that appeared to be made of shiny golden crystals...
If it was to be more magical, I'd say something akin to a warm light, a protective force that could be felt on on your skin. For the ice bits, probably twinkles of pale blue, or spots where the warmth seems to give way to powdery frost?
Golden? I'd imagine that one would be more sol than luna tbh. Hmm... Would the color of the night sky work since it's mainly formed of luna magic? Or would it be weird given the nuance?
I was thinking purple, but when I said all that my mind was thinking of the Celeste Mirror Temple barriers. Spoiler: Celeste Mirror Temple (and also a demon not getting through) barriers Spoiler: Another shot, warning for many eyeballs Something translucent like that, with the starry magic veil.
For my opinion, I would imagine imagine thousands of miniature ice crystals suspended in the air that form a barrier together. Kind of like an active defence mechanism, if you will, so that if an enemy approaches the crystals would hone in on them. If it is made of luna magic, then you might as well have the crystals appear with a dark aura. And if you really want to go for the barrier type, you could have the crystals appear as a wall or perhaps a dome. Or they could be invisible.
Hah, don't worry, there will be no eyeballs IC. @WavePearl @ChocoChicken @122 Generation Anyway, now that the main plot is up, your MCs have several options on what to do: Join the archers with Blacky and Archen; Join the attacking mages and minstrels with Celine; Help maintaining the barrier with Mawile and the barrier squad; Help evacuating people with Josson and the rest of Expedition Society; or Have an NPC from the evac squad above to take your MC to an evacuation point. The last option is there in case your MC is that tired after the last campaign.