It's an adjective: In "delusions dispelled" the participle is functioning as an adjective. This can cause confusion with language learners simply because there is a lot more emphasis and material on the role of the past participle as part of English verb conjugations.
How to decide between verb and adjective form? If a word answers "what action happened?" then it's acting as part of a verb (and this will typically be obvious as a verb conjugation). If a word answers the question "which?" or "what kind?" then it's acting as an adjective.
But it still retains a verb-like quality! The adjectival participle describes "which" or "what kind?" by virtue of the state or action it has taken or been subjected-to.
Participles as Verbs: Let's first review some familiar verb conjugations. You can easily understand these as part of the action of the sentence. "He had dispelled his delusion" and "He dispelled his delusion" both use 'dispelled' to simply describe the action (the verb of the sentence), forming the past perfect and past tense, respectively.
Participles as Adjective: In the sentence, "He was dismayed by his dispelled delusion", the word 'dispelled' describes which delusion and is therefore acting as an adjective. But even as an adjective, it describes it in a verb-like manner.
Rhetorical Usage/Elliiptical Construction: Placed after the noun (called postpostive position), participles often form an elliptical construction (omission of one or more words that are understood in context). Rhetorically, this allows brevity, euphony (pleasing to the ear), and powerful punchy phrases. This also allows an elliptical construction (dropping additional information) which creates a more passive and distant tone that can avoid naming responsibility. Therefore, this form is often used in government and business documents to sound more authoritative and less connected to actual people or groups. Statements made in the passive voice are weaker, but the missing words can make it more easily accepted (because there's less information to argue against) or hide the people responsible for making decisions.
Examples of Elliptical Constructions.
- Many of the species involved are listed internationally
as endangered. (The species involved in the study...)
- The product used was made by boiling a quantity of
hops with treacle** (The product used by the company's process...)
- Because of this the skill required is often very
challenging. (The skill required by the employee to do the job right...)
- The amount of detail given will have to be appropriate
to the type of system installed. (The detail given by the supplier's instruction manual...)
- Further efforts made are likely to fail for a number
of reasons. (Further efforts made by any military action...)
- The issues raised are more diverse and just as difficult. (The issues raised by Senator XYZ...)
- All of the ideas suggested were not feasible. (The ideas suggested by the committee...)
1. A Guide to Parts of Speech - Adjectives
Google Search "postpositive+adjectives"+participles.
2. Exceptions are idiomatic deverbal adjectives).
3. Postpositive Past Participles Used on Their Own (Furuta, Yae, International Journal of Social Science and Humanity, Vol 2, No. 6, 2012).
4. Bibliography for the quotes referenced by Faruta: (1.) R. Quirk, S. Greenbaum, G. Leech, and J. Svartvik, A Comprehensive Grammar of the English Language, New York: Longman, 1985. (2.) M. Swan, Practical English Usage, 3rd ed., Oxford University Press, 2005. (3) K. Araki and M. Yasui, Sanseido’s New Dictionary of English Grammar, Tokyo: Sanseido, 1993.
5. Semantics of adjectival participles:
participles eventives resultatives statives.