One who disillusionises; a person who removes illusions or reveals harsh truths to others (British spelling).
From disillusionise + -er (agent noun suffix). The '-er' ending creates a noun meaning 'one who performs the action,' from Old English and German origins.
This is an exceptionally rare word that probably exists more in theoretical word-formation rules than in actual use—but it's a perfect example of how English allows us to build words logically even when we rarely need them.
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