The Descent (also known as Semektet) is the defining paradoxon of the Arisen, as when they lose their power, they become more autonomous.
When first summoned, mummies are at their mightiest; inhuman, brimming with occult life force, hideous and unstoppable. But the world fights back. The effort of forcing the Judges' will onto a world and age that says they vanished millennia ago takes that searing torch of Sekhem and damps it down to a guttering flame. To be Arisen is to constantly feel the sand draining from the soul's hourglass, time dripping away like blood leaking from a slow belly wound. The longer the mummy is awake, then, the more latitude she has in her actions.
Mummies can delay the descent by absorbing additional Sekhem from Relics.