A common tool of necromancers, nether mages, and others who are privvy to the arts of the dead and undead
A simple way to a fate most dreadful.
A wet brown sack… FULL OF MAGGOTS!
In the middle of an unimportant combat with some bandits a burst of wild magic transforms all of the PCs and their opponents in to random animals and monsters. They retain their intelligence (though not, of course, the ability of speech). They can either carry on the combat in their new forms, panic, or otherwise react how they see best. After about an hour, they return, unharmed, to their normal form.