Jake Kaufman went and gave Plague Knight his own entire musical identity. The Plague of Shadows expansion for Shovel Knight reworks familiar themes with this manic, bubbling energy that fits a mad alchemist perfectly. "The Alchemist's Haven" has this bouncy swagger that feels like a villain who's having way too much fun.
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"Waltz for One" though, that's the gut punch. It's lonely and delicate and says more about Plague Knight's character than any cutscene could. Then "Le Bouquet Magique" closes things out with surprising warmth. The whole soundtrack manages to be silly and sincere at the same time.
If you played the base game but skipped the expansions, you're missing some of Kaufman's best work.