The black door on the Dreadnought requires a good amount of legwork if you're willing to do it in Sand: Raiders of Sophie.
This guide shows players where to use the Mystery Key in Deltarune.
Among the several keys you can find in Hollow Knight: Silksong is the Apostate Key. This particular key will allow you to unlock doors you might run into while trapped in bug jail, AKA the Slab.
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