The Mummy fought a ritualistic war with the Vigil in order to terrorize the many worlds surrounding Akhaten, with the Vigil sacrificing a child to stop the parasitic planet from awakening and the Mummy destined to wake the planet should the Vigil fail to bring such a sacrifice.
Indeed the Mummy was a part of the sacrifice itself, with the child singing in front of the Mummy before either sacrificing herself or being killed by the Vigil to appease Akhaten.
For many years this bloody cycle continued and Akhaten fed on the worlds around it as the Vigil seemed to be able to keep the Mummy in an eternal stalemate - until the arrival of the Doctor, who sought to end the bloodshed and confront Akhaten.
Thus when the Doctor succeeded in stopping the Vigil the Mummy ceased its long waited prize and awoke Akhaten, though in the process he would fall victim to the Doctor's greatest strength (using his enemies strengths against them) and the Doctor proceeded to confront and seemingly destroy Akhaten.
With the "death" of Akhaten the Mummy is most likely destroyed, since they were symbiotic entities.