Dear people,

Soon, my players will face off against a Mummy lord. During and after the battle they will encounter the mummy lord's staff. Which i intent to be a magical item that can be picked up by one of my players (My players are Level 11). 
Generally, a mummy lord is a cleric, but I would like for the staff to be able to cast spells that aren't necessarily on the cleric spell list. But I would like for them to be either Necromantic and/or desert themed.

What would be some good spells that I can use for this staff?

Since a Mummy Lord has access to all of these spells, I figure you can probably pick and choose which spells you want the staff to have:

Cantrips: sacred flame, thaumaturgy

1st level: command, guiding bolt, shield of faith

2nd level: hold person, silence, spiritual weapon

3rd level: animate dead, dispel magic

4th level: divination, guardian of faith

5th level: contagion, insect plague

6th level: harm

My personal recommendations are Spiritual Weapon (resembling a floating Khopesh), Guardian of Faith (resembling a mummified servant), Insect Plague (resembling scarabs), and/or Animate Dead (to revive some mummies). Now, I'd highly, highly recommend you use a system where the staff only has so many charges, and each spell consumes a number of charges equal to the spell level upon cast, using the player's Caster Stat for Spell DCs and the like. Speaking of, I'd recommend that it has to be attuned to by a caster of some sort. Now, the staff can regain a certain amount of charges each dawn, based on a dice roll, and if the players ever use the last charge, they gotta roll to see if the staff turns into a non-magical quarterstaff.

Thanks for the reply and I was indeed planning on adding some of these to the item. However, as I said, I was looking for spells that are not on the cleric spell list.

  I'm happy to say if anyone wants to play please come join....It's my first time on here lol....here is the link to my campaign  https://ddb.ac/campaigns/join/36266912054955631

Mold Earth and/or Move Earth would both be good options.  I would flavour that as the mummy has been entombed in its stone monument, which, historically, would have been sealed to prevent graverobbing. On the off chance their lord would return from the dead as a mummy, they would have been buried with something allowing them to escape their stone tomb if they so required.

Look no further than the good 'ol Brendan Frasier "The Mummy" movies for some inspiration.

Insect Plague, Animate Dead, Contagion, Bestow Curse, Harm are all fantastic ideas that have been mentioned.

Don't forget about certain desert themed spells like "Dust Devil" (eh, not so good), or "Erupting Earth" and "Wall of Sand" (VERY appropriate).