The ashes don't disappear, the task is not over yet, and we can continue to complete it later. The follow-up task will let you go to the room in front of the 3 rd to find the diary. After some conversation, you will eventually kill the demon on the rooftop, that is, Prince Malzac.
Story background (Angela invasion)
The world was almost destroyed by the black dragon prince nefario. The hero of the League saved Marshal Winsor and revealed the true face of Tatsu Yamashiro, who was actually Onyxia, the Princess of the Black Dragon. Stormwind is United again, and the alliance is more stable.
The warriors of the tribe went deep into Blackstone Mountain and bravely defeated Everett. Finally, the hero of the United Alliance defeated Nefario, the prince of the black dragon, in the nest of black wings on the upper floor of Blackstone Mountain. It seems that the world will return to peace.