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This is extrapolated off the firecracker gun just put in the Shop of Wonders yesterday, which struck me as usable as a flare gun signal. I don't know about combat balance issues, but what I was thinking was an item that allows a small message, maybe with color, to be communicated with the same range as a Yell. I am not trying to create a Sceptre of Aquaeous Mastery utility-wise. To that end, maybe make it explicitly labelled and identifiable as an illusion, to prevent it from becoming overpowered, (but not labelled in such a way that ruins its potential for decoration and signaling.)
I know illusions can be very powerful, but I am thinking more like the firecrackers in the beginning of Lord of the Rings (such as a firecracker illusion dragon swooping down) or simply a fancy communication such as "Gather for the Rite of Dawn in Silverbright Square!", except it's a fancier message. I don't mean to suggest something that's OP in combat, merely something that allows cool stuff to be communicated in Yell range with more potential for coloring and fancy decoration than the Yell command itself.
Honestly, if there's time, and the Garden likes it, maybe stick it in the Shop of Wonders for 10 crowns. We're all crown-happy right now anyway.
Thank you for reading my idea