I don’t think the dagger can be destroyed or if it can only in a very special way. The thing with magical weapons – and the dagger is one hell of a magical object – is that they are either indestructible or nearly so. They often need a special, complex, dangerous method to break.
In my opinion the Dark One’s dagger can only be destroyed by lifting the Dark One of his curse, however you do that. Lifting the curse will turn the magical dagger into a normal dagger.
This is just an assumption and there are no references in any episode. I like the subgenre fantasy and it often plays out like that. Either a very complex way to destroy it or some way to make it ineffective …
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