I know that by lore, giving an unseen servant some semblance of separateness from the caster is possible in that a published adventure contains at least one example of this occurring:
In Waterdeep: Dungeon of the Mad Mage, level 2, area 11b. Master of the Unseen (p. 35), Midna Tauberth, a priestess of Shar, is waited on by 9 living unseen servants left behind by one of Halaster's apprentices. It states:
"The unseen servants obey Midna’s commands because she reminds them of their creator. They follow Midna and can leave the room to fulfill her wishes. Midna has no way to distinguish one servant from another, meaning that when she utters a command, all the servants follow that command. She can’t issue orders to a single servant without the others performing the same task."
How can a character make these using the normal unseen servant spell and presumably something else?
(Simply saying that there isn't anything in the spell description that says whether this is possible, or simply referring to the "Spells do what they say they do" rule, is a little obvious and would be unappreciated.)
I need an answer that explains how the apprentice made these by rules or by lore.