The "charm trio" here are Loyalty, Charm,and Enslave. I've revised them a bit for my DF campaign.
First, some revision background.
Overall, the approach is pretty workable - one spell to ensure loyalty, one to ensure a much stronger charm, one to ensure total control - with no limits.
In practice they're a bit weak for their cost until the top end, when it gets extremely strong. Not only that, but they have some odd cost/casting issues.
Loyalty is probably closest to the "Charm Person" spell of D&D its descendants. The subject becomes loyal to the caster, but has a chance to break the spell if ordered to do anything against his or her beliefs or that is too risky.
What's odd about it, mostly, is cost. It's 2 to cast, 2 to maintain. Double cost if the subject doesn't know the caster, triple if it's a foe. Those types of costs aren't forbidden in GURPS Magic and it's not an issue here, until you look at Charm. Charm is a flat cost on anyone, and it's 6/3. So except for non-hostile people who know you Loyalty isn't such a great deal. The costing based on familiarity is something I don't really like - skill penalties for people who don't know you, sure. That's how a number of other spells work.
Duration is 1 hour. Not bad. It oddly takes Bravery as a base spell, which means Sense Emotion->Fear->Bravery before you can charm people. Okay, but oddly again Loyalty is a prereq for Emotion Control, which is Bravery with less specialization. What a mess. I'd think Emotion Control would be a better prereq for Loyalty than the other way around - before you can make someone your loyal friend, you have to be able to make them friendly. That's a messy fix, though, but I do think it's potentially useful to think of Loyalty as needing a weak will to get through.
Now we're getting some serious charming. This isn't "loyal friend" but "faithful slave" and no risk is too great.
The cost is odd compared to Loyalty. Why not 6/6? It's better than Loyalty cast for 6/6. It's only a 1 minute duration, which makes all the nonsense about being unwilling to discuss loyalty to the caster and such less important. It's not like there is a boatload of Charm-25 folks out there who can keep this up on anyone for any length of time.
It's one second longer to cast, which is good.
This is the top end charm. It's permanent, it's flawless, it's only 30 points, and it comes with a mental link thanks to its Telepathy prereq.
I never liked that Enslave gives so many extra benefits - you get a flawless eyes-and-ears possession out of it with none of the usual costs and problems of such a spell. It also has a flawless telepathic connection. Instead of being a permanent and very strong charm, it's that plus a whole side benefit of a permanent mindlink that's actually better than Telepathy can give you. A fully loyal subject means you can put such spells on the target with ease at another time - it doesn't need to come with them in this over-amped package.
There used to be a whole thread - on GURPSNet-L or the forums, I can't recall - that basically supposed a magical military force would be based on using Enslave to link everyone together into a cohesive whole. Yeah, that'll work - and who relinquishes total personal control of the military? It sounds more like a great explanation why the Great Witch-King of Blackguardia controls every single person in his whole territory. Good campaign backstory, iffy idea in practice. Or at least, if you assume the guy with everyone Enslave doesn't just rule the whole world.
Oh sure, it's resisted by Will. But you don't have to choose to resist. Best way to Enslave? Cast Loyalty or Charm, and then tell them not to resist your spells. They'll do it. Cast and done, permanent slave.
Anyway, I like the idea of a permanent charm. I like the idea of lasting charms, too. So why not just make that a feature of the spells?
I also like the idea of a lasting Charm - that is, you put the spell on and it lasts until it fails a test.
Here are the revised version from my current game:
As written, but for a higher cost, duration can be made Lasting A lasting version of the spell can be broken with an IQ roll just like the temporary version. A subject who breaks the spell will be mentally stunned (p. B420) for one second, then recovery automatically.
Duration: 1 hour or indefinite.
Cost: 4 to cast, 2 to maintain. Cost 10 to make the spell Lasting, (per GURPS Magic, p. 10).
Prerequisites: Magery 1 and Bravery and two other Mind Control spells or Weaken Will and two other Mind Control spells.
As written, but for a higher cost, duration can be made lasting. If the subject is told to do something diametrically opposed to his or her moral code, or directly suicidal ("Kill yourself!"), the subject may attempt an IQ roll - on a success, the character will refuse the command, but is still subject to the charm spell. Treat this as being mentally stunned (p. B420), but recovery is automatic after one second.
Duration: 1 minute or indefinite.
Cost: 6 to cast, 3 to maintain. Cost 15 to make the spell Lasting, (per GURPS Magic, p. 10).
Prerequisites: Magery 2 and Bravery and two other Mind Control spells or Weaken Will and two other Mind Control spells.
As written, but remove the mental link. The subject does not receive an IQ roll to break the spell. Treat the subject as the faithful slave of the caster.
Prerequisites: Magery 3, Charm, and 10 other mind control spells.
Limitations on Control: A caster can only have permanent control (via Enslave) of a limited number of subjects. Find Magery level in the "Size" column of the Size and Speed/Range Table (p. B550) and interpret "Linear Measurement" as "permanently enslaved subjects" instead of "yards." This means:
Magery 3: Seven Enslaved subjects.
Magery 4: 10 Enslaved subjects.
Magery 5: 15 Enslaved subjects.
Magery 6: 20 Enslaved subjects.
There is no limit on Lasting casts of Loyalty or Charm.
Can I cast Loyalty on someone, and then order them to not resist Enslave?
Sure. That's the way to do it. That might seem like it makes it cheap to make slave armies, but it's no more powerful than "Can I kill someone and then Zombie them?" or "Can I put someone to Sleep and then cut their throats" or "Can I Levitate someone who thinks I'm his friend and then drop him off a cliff?" A slow gateway spell into a permanent one is pretty common, and charm-type spells aren't so powerful they need a special carve-out to make them weaker.