What is the most powerful permanent undead follower you can get?
Any class/race/spell/level combo is fine. Is it as simple as a necromancy wizard controlling a mummy lord, or is there something better we can do with other options?
Constraints/criteria:
Exclude wish.
Magic items are ok – as rare as you like, but not including homebrew items.
Making yourself undead doesn’t count.
"Most powerful" is definee most powerful by challenge rating; but …
… if you come up with some ridiculous DPR combo, list that too.
Best Answer
CR 23
Specifically, you can permanently enslave Acererak, the demilich, in his Tomb of Horrors. No undead has a higher CR so you can't really do better than that1.
To enslave Acererak, one ought first to Feeblemind him and apply several high-level copies of Bestow Curse2. This is not easy, as Acererak has a +11 bonus on Int saves, a +9 bonus on Wis saves, and 3 free saves per day from legendary resistance. Still, it is possible, and not even terribly difficult for a 20th level wizard with access to Bestow Curse, Glyph of Warding, Feeblemind, and Gate (though it will be quite expensive).3
Once Acererak is reduced to 1 intelligence and charisma, given disadvantage on Charisma and Wisdom saves, and made to make a Wisdom save on each turn or lose its action, you should have allied casters with access to bane (e.g. a drider or Koa-Toa whip created via true polymorph and controlled via dominate person via glyph of warding to bypass concentration) cast bane on the demilich till it sticks. This gives Acererak a charisma save of 1d20+1-1d4 with disadvantage. As a 20th level wizard with 30 intelligence4, your save dc should be 24. With a Robe of the Archmagi, your DC goes to 26. This means that even with a 20 Acererack is unable to resist your Command Undead class feature, and becomes your slave.
Since a feebleminded demilich is much less useful than a functional one, we next need to deal with that. Greater Restoration (accessed via true polymorph on an object into a Coautl, combined with dominate monster via glyph of warding if necessary) eliminates the Feeblemind without hurting the rest of the debilitating effects.5
Immediately after restoring the demilich's mind, order it to use its teleporting randomly within a 300 mile radius and lowering a stat by two permanently ability on itself choosing intelligence until its intelligence score is 10 or 11. This must be achieved within 1 hour. Once the lich's int score dips below 12, he stops being entitled to hourly saves and instead serves you without any further saves forever (or, at least, until control is actively lost). If a save is incurred during or as a result of this transition period, it is 1d20 +11-1d4 without advantage versus DC 26. That's about a 17.5% (a little less than 1 in 5) chance of us failing, which we can mitigate by forcing a reroll with a Luck Blade for a total of 3.06% (a little less than 1 in 30) chance of losing the demilich permanently per roll. 6
Note that, in practical terms, regular liches make better servants that Acererak does since they can move and cast spells and Acererak can't really. Acererak has a higher in-lair CR, though.
Okay, actually ancient red or gold dracoliches are CR 24. But 1) Acererak is way famouser, and 2) the same technique (except lowering via other methods, unless you have an Acererak sitting around) would work on a dracolich in the same way, it's just easier because Acererak has a +11 intelligence save instead of a measly +4. In fact, dracoliches can be easily taken down with just a horde of intellect devourers, which is sad. Reminiscent of earlier editions, but still sad.
To capture a dracolich variant spellcaster, have a horde of intellect devourers you control set its intelligence to 0 and leave it permastunned as soon as you release the lich from Feeblemind, which you do after Polymorphing it into a swarm of rats or similar. Since the rats are immune to stunning, the Dracolich will have 0 int without the penalty of being stunned. Undoing the polymorph doesn't remove the effect setting Int to zero, but it also doesn't cause the lich to be stunned. Dracoliches, like all dragons, use Charisma as their spellcasting attribute, so 0 int isn't a big deal. Alternatively, you can use a scroll of Power Word Heal, if you happen across one.
Bestow Curse may stack weird in some games, see this Q+A If it does, you'll need to cycle through curse effects to make this work. You can probably do that just by redefining what curse effect is 'more powerful' repeatedly, but worst comes to worst you can spend the months of time and tons of gold to make the necessary Remove Curse and Bestow Curse cycles operate via Glyph of Warding.
You do this by casting Glyph of Warding a bunch of times with Feeblemind in it, obviously. You also have some Glyphs with Bestow Curse and maybe some other debuff-y spells too if you want. You can't put Bane in here, cause you don't know it cause it's not a wizard spell. You then take a long rest, and cast Gate on the last round of said long rest so that you immediately regain all your spell slots and can cast a 9th level spell again right away. This works because it takes an hour of interruption to stop a long rest from completing. Since you need to be sure the lich fails the save four times, you need a lot of Feebleminds. Specifically, you need 15 Glyph of Wardings with Feebleminds to have a 99.99% chance of success. That costs 3000 gp to create and takes 15 days to create for a 20th level wizard. You also want the curses to stick which means, to be safe, spending another 11400 gp but no additional days to the contruction time.
If you have a Tome of Clear Thought, you and anyone you want have 30 Intelligence, assuming you're willing to pass time for the book to recharge, for example by putting the book on a plane where time moves faster or by going to a plane where time moves slower or both, or by entering temporal stasis, or by finding a way to magically age the book or whatnot. Regardless, if you have 5 Tome of Clear Thoughts, you have 30 int.
If you, for some reason, didn't or couldn't track down a Tome of Clear Thought by level 20 as a wizard, your intellect will probably be limited to a mere 20, which puts your save DC as a mere 19 or 21 with a Robe of the Archmagi. Don't do this to yourself.
We can't use glyphs for True Polymorph, because the spell is 9th level and we only have 1 9th level spell slot, unless we also have a Tome of the Still Tongue. That's why we may need to, instead, create creatures on prior days and cage them, then use Glyphs to Dominate them on the last day. We probably shouldn't need to do this, because we probably have 30 Charisma and no reason to treat our servants badly and they have no reason not to obey us, but maybe we do or we're super evil or something. Setting this up adds weeks of time and tens of thousands or gold pieces in expenses if you decide to do it the Dominate-y way.
We need the Coautls (or similar) because Greater Restoration is not on the Wizard list, and that spell or Heal are needed to remove the Feeblemind without removing other things we want to keep.
Admittedly, the only benefit Acererak derives as a servant from not being Feebleminded is that he can communicate intelligibly and has a +5ish bonus on Int and Cha saves. Nonetheless, in the case of a proper lich Intelligence is a very important stat so understanding how to undo the Feeblemind without losing control is important in general.
The intelligence is lost when the curse is removed, which you can do by casting Remove Curse. You'll need to track down or gate to you or otherwise undo the random teleport's separation 5 times within an hour to avoid letting the lich get a save.