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A blend of the nouns demon and menagerie, the rare noun demonagerie designates a menagerie of demons.

The only occurrences of the noun demonagerie that I have found are as follows, in chronological order:

1-: From a review of Memoir of William Ellery Channing: with extracts from his correspondence and manuscripts (London: John Chapman, 1848)—review published in Tait’s Edinburgh Magazine (Edinburgh, Midlothian, Scotland) of July 1848 [page 433, column 1]:

His grandfather was a merchant of Newport, a slave-owner; and the following extract, affording a glimpse of days long gone by, in the northern states, is of interest. The slave population in the middle of the last century, a hundred years ago, occupied a position superior to their present circumstances. Slavery deteriorates the buyer and the bought, and unless it had been now and then checked, would have transformed the earth ere now into a demonagerie.

2-: From A New English Dictionary on Historical Principles; founded mainly on the materials collected by the Philological Society (Oxford: The Clarendon Press, 1897), edited by the British philologist and lexicographer James Augustus Henry Murray (1837-1915) [Vol. III, Part 1: D, page 185, column 1]:

Demona·gerie. nonce-wd. [f. Demon, after menagerie.] An assemblage of demons.
1848 Tait’s Mag. XV. 433 Slavery . . unless it had been now and then checked, would have transformed the earth ere now into a demonagerie.

3-: From Black Sun Rising (New York: Daw Books Inc., 1992), by the U.S. author Celia S. Friedman (born 1957) [chapter 34, page 383]:

At night the demons would come out, bloodsuckers and their kin—but they were mild creatures compared to the denizens of the Forest, or even Jaggonath’s demonagerie: constructs of the party’s fears that had manifested enough flesh to make a fleeting appeal for sustenance, but little more than that.

4-: From Michael Gartner’s Words column, published in The Courier-Journal (Louisville, Kentucky, USA) of Sunday 7^th February 1999 [page D2, column 4]:

All demons are guys. Lady demons are demonesses. As long as we’re this far into demonology—that’s the study of demons—I might as well tell you that a demoniast is a person who has dealings with demons, the demonarch is the chief demon, and a demonagerie is an assembly of demons.