Back to Hogwarts to Destroy the Last Horcrux

In this pivotal moment Aberforth Dumbledore shows Harry, Hermione and Ron a way back to Hogwarts (without being detected) which is hidden behind the painting of Ariana Dumbledore. Ariana leaves and comes back with Neville Longbottom, who escorts them through the hidden tunnel back into the castle to reunite with the rest of Gryffindor House. This is the most unique and singular way Harry and his friends enter the school and it is the one time where he enters the school through this hidden entrance to initiate the final battle against Lord Voldemort.