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:
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 think you're going to be shocked when you take it out of the white room lmao
It's situational. I know that. But that's why it's fun to come up with these really weird build ideas. This one just so happens to be more effective against the players than the enemies they fight, for the most part, part of why I noted it would make a great villain.