Lost Menagerie (5E)

From publisher blurb: They say favors make the world go around, and it’s true of the planes of existence beyond material reality as well. In the distant past, the great wizard Arfaleo the Seeker claimed favors from four mighty elemental lords to create a self-contained world. The wizard populated his microcosm with strange creatures, to monitor what changes might occur to them over time, even bringing a small settlement of humans into the interplanar cavern. Eventually, though, the wizard’s interest in his experiment waned and he moved on, leaving behind the bizarre anomaly he had created. Since this time, the microcosm has changed in many ways, for it did not remain isolated from other planes of existence. New creatures have made their way into the wizard’s carefully-maintained experiment, and — there is no other way to say it — things have gone very wrong. Lost Menagerie is a challenging adventure for characters of 7th to 9th level.