The terrifying Borean Commander Mouse has a mimetic ice-alloy body, consisting of an interesting form of "liquid ice"-like nitrogen. This allows for her to change her form and create different shapes out of her icy structure.
Leading the mice in constructing a lair of pure ice as a base of operations, she may be trying to build a permanent home to fend off the warmer weather so she does not have to disappear with the season.
Though she is currently constituted in the overall shape of a mouse, she may not even be one at all!
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