The Marauders shared a strong bond. On finding out that Remus Lupin goes through an excruciating transformation to become a werewolf (Lycan) every full moon, the quad secretly took on their Animagi form to contain him. James turned into a stag (Prongs), Sirius Black turned into a dog (Padfoot) and Peter into a rat (Wormtail). Their Animagus forms helped them explore the outside grounds and with James' Invisibility Cloak, they explored every nook and corner of Hogwarts. Using this knowledge, the four friends created the Marauder's Map to record all the secret passages, which helped them create mischief.
The Marauders were bold enough to put their Animagi names on the map even though it was illegal for Hogwarts students to perform unsupervised magic. The map is proof of their advanced magical ability. The Marauder's Map is enchanted with 'Homonculous Charm' spell to track the movements and 'Forever Repel' spell for Snape, James' arch-nemesis.
The map only reveals itself when you tap on it gently with your wand and say "I solemnly swear that I am up for no good". Once you're done using it, you tap on it again and say " Mischief Managed" and it turns into a blank parchment paper. In their last year of school, it was sequestered by Argus Filch, the caretaker of Hogwarts, probably after a tip-off from Snape. Filch wandered the corridors and passages of Hogwarts with his cat Mrs. Morris to catch students breaking rules.
Years later, the map was stolen by the Weasley twins, Fred and George, the older brothers of Ron, who was Harry's best friend. They reckoned that Harry needed the map to find Voldemort, who was living and plotting within the castle. So they unknowingly passed on the map to one of the Marauder's sons, Harry Potter.
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