Dumbledore's Army
An underground club in Hogwarts that learned battle magic when it was banned by the school, They went on to turn the tide in the Battle of Hogwarts
Where are they from?
The creation of Dumbledore's Army was a major plot point in the fifth Harry Potter novel The Order of the Phoenix which was published in 2003, and then adapted into a film in 2007. When the students are forbidden to learn battle magic they turn to Harry. Dumbledore's Army, or DA, wound up being a major part of the defense of Hogwarts as the series continued.