At the beginning of the First Wizarding War, Lord Voldemort secretely ordered Death Eaters Bellatrix Lestrange and Severus Snape to attack Azkaban Wizarding Prison and release every mass-murderer imprisoned, and dementors all over Muggle world to terrorize the people. However, Snape was secretely acting on behalf of Albus Dumbledore, the greatest wizard of all times and the main enemy of Voldemort's. Dumbledore wanted Snape to obey Voldemort and to successfully fullfil his wishes in order to convence him of his loyalty.
The raid was led by Lestrange and Snape successfully, with the dead of every security auror of the prison and the Chief of them all, Robartus Gorllinton. Many prisoners were released, though many of them were killed by the aurors to prevent their escape, and all of the dementors fled, being guided by a control charm made by Lord Voldemort. Voldemort guided all the dementors through the sea and released them all over the muggle populace in London. However, the dementors were later aprehended by Albus Dumbledore, Alastor Moody, Remus Lupin, James Potter and Severus Snape himself.