Choose an undead creature you can see of CR 4 or lower within range (taking 2d10 necrotic damage per CR of this spells target before the save is rolled). It must make a DC 20 Wisdom saving throw. On a failed save the creature is under your control for 24 hours, after which it stops obeying any command you've given it.
As a bonus action on each of your turns, you can mentally command any creature you control with this spell if the creature is within 120 feet of you (if you control multiple creatures, you can command any or all of them at the same time, issuing the same command to each one). You decide what action the creature will take and where it will move during its next turn, or you can issue a general command, such as to guard a particular chamber or corridor. If you issue no commands, the creature only defends itself against hostile creatures. Once given an order, the creature continues to follow it until its task is complete.
To maintain control of the creature for another 24 hours, you must cast this spell on the creature again before the current 24-hour period ends. This use of the spell reasserts your control over it.
This spell cannot be stored by any means other than inside the Orb of Undead Servitude. Attempting to store this by other means will be unsuccessful and cause the attempting creature and the Orb of Undead Servitude to cease to exist.