Sirius Black dies amid a battle in the Department of Mysteries when his godson, Harry Potter, and other members of Dumbledore's Army are trying to prevent Death Eaters from getting hold of the prophesy which foretold what must occur between Harry and Lord Voldemort.

Initially, Lord Voldemort had caused Harry to have a dream that Sirius was in the Department of Mysteries to lure Harry there. The plan worked, and Harry only realizes during the struggle that Sirius wasn't there.

Once battle breaks out between the Death Eaters and Dumbledore's Army, members of the Order of the Phoenix, including Sirius, arrive and enter the battle. Later, Dumbledore arrives, and the only wizards left dueling are Sirius and Bellatrix Lestrange. To his great detriment, Sirius urges Bellatrix to perform better, and her next spell hits him directly in the chest. Harry and the others watch in horror as he disappears into the veil behind him. Harry runs forward to help, but Lupin grabs him, telling Harry that there is nothing he can do.

Prior to the release of Harry Potter and the Half Blood Prince, there was much speculation as to whether "disappearing into the veil" did actually mean death. Sadly for Harry, his godfather was indeed dead, having been back in his life for just two years.