Can't really be a Koopa if you haven't been undead at least once in your "life," right? Alas, that's the downside of having transformative powers, even if you can only normally switch between two forms without other magical assistance, they'll still find some way to mess you up!

Kind of a continuation of this:
https://www.weasyl.com/~featheryoshi/submissions/2248196/bones-enning