The marauders + A few

Ariana Riddle was the half sister of the one and only Tom Riddle, but this story isn't about her, no it's about her best friend, the twin of Sirius Black. Cassiopeia Walburga Black.
The two Gryffindors were the mischief makers of Hogwarts and whenever anyone asked Cassy what she did that day she would reply along the lines of "Pranked people, got told of, pranked Regulus, got told of, went flying with Siri on his new bike but mainly all the same things over again"
But what will happen when Cassy realizes that some of the most important people in her life, are in mortal peril? Join Cassy, Alice, Frank, Molly and the marauders travel through the mind chamber to try and save those poor people. Only, the question is Who are they?