A paradelle is an incredibly demanding fixed form of poetry concocted by Billy Collins in late 1990s as a parody of extremely confining and repetitive forms of poetry, in particular the villanelle.
Collins had grown tired of the recent revival in popularity of old forms such as the sestina and the villanelle, which he found to be repetative, sing-songy, and just plain dumb. So in 1997 he published a poem called "Paradelle for Susan" and in a footnote wrote that it was an ancient poetic form dating back to French love poetry of the 11th century. Collins' poem was intentionally ridiculous, and included lines such as "Darken the mountain, time and find was my into it was with to to."
However, much to Collins' surprise, numerous people did not get the joke, and his description of the form and its origins has been replicated numerous times in quite serious contexts. Moreover, the form actually became quite popular over the next decade, both among people who didn't get the joke but also even those who did, perhaps because it represents perhaps the most extreme challenge in poetry, kind of like an X Games version of the villanelle. As Collins wrote in a recent retrospective:
"The paradelle invites you in with its offer of nursery-rhyme repetition, then suddenly confronts you with an extreme verbal challenge. It lurches from the comfort of repetition to the crossword-puzzle anxiety of fitting a specific vocabulary into a tightly bounded space. While the level of difficulty in most verse forms remains fairly consistent throughout, the paradelle accelerates from kindergarten to college and back to kindergarten several times and ends in a think-tank called the Institute for Advanced Word Play. Thus the jumpy double nature of the paradelle, so unsteady, so schizo, so right for our times.
By 2005, there were even enough good paradelles in circulation that someone was able to publish an anthology of great paradelles (Red Hen Press: The Paradelle, ISBN: 1-59709-023-9).