Walpurgisnacht is a particularly tough enemy from the Madoka Magica collab. This is because her falling building attack can destroy lots of cats at once and slow down any survivors. Her area of effect is tremendously wide as well. Also keep in mind that she is not a Floating enemy despite being in the air.
Walpurgisnacht is also the name of a German holiday. Walpurgis is a celebration commemorating Saint Walpurga starting on April 30th and ending on the 1st of May, while Nacht means night in German. Walpurgis is commonly associated with religion and celebrations and, like many things that are associated with religions, also with witchcraft.
Walpurgisnacht was far more powerful during the original run of the Madoka Magica collab in the Japanese version, boasting tremendous attack power.