Here are the few themes that I think we can use for Witherstruck.

For Witherstruck Y's theme, "Heartache".

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For the Witherstorm battle, "Battle Against a True Hero".

[There should be a GIF or video here. Update the app now to see it.]

Random themes: Omega Flowey (Finale) and Arisel Dreemurr (Hopes and Dreams)

[There should be a GIF or video here. Update the app now to see it.]

[There should be a GIF or video here. Update the app now to see it.]

[There should be a GIF or video here. Update the app now to see it.]

That's all the themes I can think of for now. Might add more.

See ya!