Mu dies during the final fight?
It’s well know that power corrupts, and that absolute power corrupts absolutely, especially when it comes to magic, but it’s not often discussed how someone handles that corruption.
Not everyone manages to handle it as well as Subcon’s residents do, and with the volatility of temporal magic, the risks increase exponentially.
Mustache Girl doesn’t realize the power she’s gained from the time pieces is burning her being from the inside out, until partway through her fight with Hat Kid, when she starts glitching uncontrollably across the arena.
This ramps quickly and Hat Kid, fearfully recognizing what’s happening, uses the last time piece she got before Mu stole the rest, to get everyone out of there and on her ship asap
For a few moments nothing seems to happen, though everyone is disorientated from the sudden trip, but then a blindingly bright light starts to shine from Mustache Girl’s castle, growing to encompass the entire planet and it’s moon.
When the light finally dies down, and everyone aboard looks to see what happened they find nothing. No planet or moon or anything, just a void where it used to be.
Hat Kid somberly explains what happened, how that amount of temporal magic, outside of containment created a sort of temporal black hole, wiping their planet from time itself. The only thing that kept them safe is the temporal shielding her ship has.