As above - the mummy starts gathering minions, setting up traps to deter intruders, using divinations to orient itself, and determining what it's going to do next.
It's a mummy lord, an intelligent and OLD undead. What does it want, in the context of your game?
-Is it a researcher of some sort? Astronomy? Alchemy? Arcane symbols?
-Does it just want to kill people and make them undead?
-Does it want to shut down necromancy and secure dead bodies as dead?
-Does it want to become a new pharoah?
-Who does it worship? In past editions of D&D, mummies with magic often had clerical magic.