The usage is perfectly grammatical. Rampant is an adjective in the "sentence". In other more inflected languages such as Finnish there is a special grammatical case -- the essive -- which is used in contexts like this.
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English 1b3Aren't we needing an adverb here? How is it running?I hate to reiterate, but rampant does not indicate how something is running. It shows what something is likewhen it is running. If you think differently, that is of course fine with me!
English 1b3If 'rampant' is an adjective, where's the noun it modifies? What do you suggest it is modifying?Nothing, because you didn't give us a complete sentence.