As others have pointed out, slave magic (as well as all other forms of psychological magic) is severely underdeveloped in anime. No one seems to understand the depths of the rabbit hole they open up.
I'll focus on slave magic. First, think about the slavery we have in our world. Why does it happen? How do people enforce it? Why do people submit to it? Most importantly: Are slaves still human beings with their own individuality?
Now, think about your average slave magic. It doesn't particularly matter the details, but there are four main points that give rise to absurd power:
a. A master can enslave a victim whether or not the victim is willing (pretty average stuff)
b. A slave must obey all commands that are given to them by the master (wow, slaves)
c. There is no limit on the number of slaves a master can own, or the limit can be expanded with relative ease (I don't think any anime ever mentioned a limit)
d. A slave can enslave a victim for the master or for themselves (While not in any anime I know of, no anime doesn't explicitly disallow this either. Either way, this is the fun point)
To bring the strength of this magic into perspective, connecting it to our own world may help. Imagine you are the boss of a moderately-sized criminal organization in our own world. One day, you gain slave magic (e.g. You learn it somehow, you find a box with infinite slave collars, etc.). Think, how would you use this power for your own personal gain?
Here's a checklist for conquering the world:
1. Enslave the organization's inner circle. Already you have extreme power. The top brass cannot betray you, cannot spill secrets, cannot lie, cannot leave, they will follow every one of your orders perfectly.
2. Enslave the rest of the organization. You now have total control of the group. By ordering your slaves to generally act normally, family members won't detect anything, police will experience no change, even the slaves may not know they're slaves.
3. Enslave the local populace, particularly people in positions of power. Order them to enslave more people.
4. Enslave government officals. Arrange meetings with celebrities or other influencers. You now have total control of the country.
5. Enslave other countries. Send out assassins, not to kill, but to enslave. Take control of whole countries of people, their officials, their celebrities, their rebels, their extremists, their sons and daughters.
6. If an organization attempts to resist, crush them. Demonstrate your power in front of the international community and issue an ultimatum: "surrender or I will order millions of men and women, youth and elderly, healthy and diseased, to charge at your cities and fortresses."
7. Nothing can oppose you. It would be extremely difficult to detect slave sleeper agents, riots across the globe may be breaking out, and an ungodly humanitarian crisis may be about to unfold. Governments, organizations, institutions, everything folds.
8. Congratulations, you are now god-emperor of the world! No one can disobey you, no one can rebel, you can order billions to play out whatever fantasy you want.
Imagine a world where everyone has this kind of power.
In our world, slaves at least had some form of agency. They obeyed orders due to fear of punishment, of death, or because of previous torture. Slaves had some potential to resist, to rebel, to disobey, to die fighting. Slave magic can override all of that. One obeys because they were told to. Slave magic denies the free will of a person, violates the sanctity of the mind. Instead of their own individual person, a slave is no more than an extension of the master's body.
Without hard limits on its power, slave magic has the potential to completely uproot society as we know it. Even with significant limits, slave magic still can have power. For example, even without any of the four above points, small cults and other extremist groups can utilize it's power to garner even more fervent following from their followers. Imagine the KKK, North Korea, the Khmer Rouge with the ability to gain completer control over their subjects.
Slave magic is one of the most potent forms of magic. Unlike other magics which have mainly physical effects and can only dominate with force and suppression, slave magic acts on the mind and can grant one the most loyal followers of all.
it is a shame that slave magic is only used for slavery.