I came across Create Thrall when I was making my character for Curse of Strahd. I decided to go with Goolock for thematic reasons, and made a list of every way to incapacitate someone in 5e, even knowing the campaign doesn't get to level 14. Hypnotic Gaze still stands out to me as the clear winner for being recyclable and also a really strong ability on its own, as well as not (RAW) breaking Sanctuary.
I still think Goolock is weak (this can't affect non-humanoids, which I find a little disappointing), but nowhere near as weak as before. Honestly, this could make a GREAT villain.
I'm not sure why you think this is any good or would require anyone to rebalance an encounter at level 16. The charmed condition is just:
A charmed creature can't attack the charmer or target the charmer with harmful abilities or magical effects.
The charmer has advantage on any ability check to interact socially with the creature.
Like okay, a single humanoid in an encounter has to attack a different party member for two turns worth of actions?
Hypnotic Gaze is such an awkward way of achieving this as well. You've invested two levels in a class with little synergy with your main for an incapacitate that only works with 5 feet of you. Your DC for this is going to be terrible as it's INT based not CHA based. You'd be better off just taking Hypnotic Pattern as a spell.
The charmed condition from Create Thrall lasts until otherwise removed, the ability is used on someone else, or a Remove Curse is cast upon the target. These abilities have no limit on daily use other than Hypnotic Gaze being once per creature per long rest.
Importantly, Create Thrall's charmed condition does not break under normal conditions, like taking damage. And if you've reached a level 14 warlock, you likely have a 20 in Charisma, which, if you have advantage on attempts to persuade the target to not attack you at that point and proficiency, you're likely going to succeed.
Finally, putting a few levels into wizard gives a warlock extra spell slots for longer encounters, which can save them. They don't want to be using 5th level spell slots on something like, Silvery Barbs, for example, despite how good it is. I'd argue taking 3 levels in wizard is even better, since your potential spell slot access doubles.
This is very much a build that can take down major threats single-handedly, and is designed to function as such. A little bit of a meme build, but an enjoyable thought experiment.
I seriously don't get what you are trying to accomplish with this awful combo and why it would require you to rebalance the encounter.
For reference:
Hypnotic Gaze
Starting at 2nd level when you choose this school, your soft words and enchanting gaze can magically enthrall another creature. As an action, choose one creature that you can see within 5 feet of you. If the target can see or hear you, it must succeed on a Wisdom saving throw against your wizard spell save DC or be charmed by you until the end of your next turn. The charmed creature's speed drops to 0, and the creature is incapacitated and visibly dazed.
On subsequent turns, you can use your action to maintain this effect, extending its duration until the end of your next turn. However, the effect ends if you move more than 5 feet away from the creature, if the creature can neither see nor hear you, or if the creature takes damage.
Once the effect ends, or if the creature succeeds on its initial saving throw against this effect, you can't use this feature on that creature again until you finish a long rest.
Create Thrall
At 14th level, you gain the ability to infect a humanoid's mind with the alien magic of your patron. You can use your action to touch an incapacitated humanoid. That creature is then charmed by you until a Remove Curse spell is cast on it, the charmed condition is removed from it, or you use this feature again.
You can communicate telepathically with the charmed creature as long as the two of you are on the same plane of existence.
A couple things stand out to me:
you are postponing your 8th and 9th level arcanum for a very shitty two-turns combo in a game where the vast majority of fights end by turn 3.
Hypnotic Gaze requires your wizard DC, but you dumped your INT because you are a Warlock. If you didn't, your stats probably suck.
Seriously, if you really, really want to thrall someone, just ask your resident fighter to deal a non-lethal blow when they defeat the enemy on turn 3 and use the thrall feature that way instead of... Whatever this thing is.
this combo is not broken and doesn't require you to rebalance your game in any way, shape or form. I'm in the camp of "I hate spellcasters because they break the game" and even I would not have a problem allowing this in my table. Your GOOchantment abomination could just take another level in warlock, learn Glibness and pass whatever mission-critical CHA check that way, or Dominate Monster if you want an actually useful charm effect mid-combat, or, I don't know, Feeblemind? Sure, this combo is technically reusable, but realistically, what monster are you fighting that's consistently failing a DC 14 WIS check at level 16? Like, is you really, really don't want your party to pull this off for whatever reason, just use a legendary resistance (or fudge the dice).
You mentioned using this combo for a villain? Warlocks also get Power Word Stun, and I doubt most of the party members have more than 150 HP at level 16. Again, combats in this game last 2-3 turns on average. Having your party member stunned for 1-2 turns immediately is better than having them charmed on turn 3. Or cast Maddening Darkness and turn everyone blind. Or pick literally any other spell in your arsenal. You are a level 16 warlock. This thing is barely passable for a level 3 midboss.
Now I feel like me and my friends are playing dnd wrong. Ending encounters in 3 turns is very rare for us. Usually they last like ~6 turns. Sometimes more, but 3 and less is extremely rare for us.
Anyway, my figure comes from the DMG, which recommends three rounds as the average length of a fight. Your mileage may vary, but that's usually accurate in all the tables I've played in, and the few I have mastered as well.
This is true, although ways to do this in combat are limited. The torpor poison is the only non-magical way to do this if you don't count unconscious as being asleep (which, technically, is a better description), although all 3 other poisons inflict this condition, as well as Eyebite.
It's true you can just wait for the target to be asleep, but the point is that you can do this mid-combat over 2 turns and there really isn't much anyone can do about it.
They could switch to support. They can't hurt you, but that doesn't stop them from buffing their allies. By RAW, they can also use area effect spells and hurt you without targetting or attacking you. And they could do help actions to give allies advantage when attacking you.