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This is the noun phrase:

... the most complicated and sensational of its kind

The possessive pronoun its refers back to "the case".

Depending on how you analyze it, in the most complicated and [most] sensational, the superlative adjectives are substantives, acting like nouns, or there is ellipsis of an understood plural "cases".

The electric car known as The Zoomer is one of the most complicated and sensational of its kind.

If you analyze it as a case of ellipsis:

The electric car known as The Zoomer is one of the most complicated and sensational cars of its kind.

If you analyze the adjectives as substantives by virtue of their being part of "its kind":

The electric car known as The Zoomer is one of the most complicated and sensational of its kind.

its kind would refer to the kind of the car known as the Zoomer, namely, "electric cars".

The electric car known as The Zoomer is one of the most complicated and sensational of electric cars.