Dissendium

From UnknowableWiki

Dissendium is the spell used to reveal the passageway leading to the Honeydukes basement, which is hidden under the one-eyed witch statue at Hogwarts. Harry Potter found out the incantation with the help of the Marauder's Map.

This charm is used almost every time someone sneaks off to Hogsmeade, such as Harry's first visit to the village (PA10), Fred and George's trip to Hogsmeade to get food and beverages to celebrate a victory against Ravenclaw (PA13) and Harry's second Hogsmeade visit (PA14). The only other passageway out of the Hogwarts castle that is not blocked by the time Harry receives the Map is the one that leads to the Shrieking Shack via the tunnel underneath the Whomping Willow (PA10).