Niv-Mizzet, as a Dracolich: CR 26
First, can Niv-Mizzet be a Dracolich?
The only requirements to become a dracolich is to be:
an ancient or adult true dragon
... and Niv-Mizzet is described as (p. 240 of Guildmaster's Gude to Ravnica):
the ancient dragon who founded and continues to control the Izzet League.
Then all you need is for Niv-Mizzet to:
allow [himself] to be transformed by necromantic energy and ancient rituals into [a] powerful undead [dracolich]...
If Niv-Mizzet recognizes that he may be killed soon (possibly by the massive party and army you have been amassing to defeat the him and the Izzet League), it would certainly be in his character to undergo such a transformation. The Monster Manual states...
Only the most narcissistic dragons choose this path, knowing that by doing so, they sever all ties to their kin and the dragon gods.
... and Guildmaster's Guide to Ravnica says that Niv-Mizzet is:
Possessed of arrogance and vanity that matches his vast intellect and tremendous power
How to turn an undead Niv-Mizzet into a permanent undead follower
This procedure hinges around the Intellect Devourer's Body Thief ability. Essentially we need to land a true polymorph on Niv-Mizzet to turn him into a humanoid and then steal his body with the Intellect Devourer. Then once we end true polymorph, the Intellect Devourer will control Niv-Mizzet in all his undead glory.
The best character to pull this off is a bard 18/warlock 1/paladin 1 with the following spells...
- true polymorph
- bestow curse
- feather fall
- enhance ability
(alternatively, you could learn find greater steed as a Magical Secret and not require a level of paladin leaving space to be a bard 17/sorcerer 3 for Heightened Spell Metamagic)
... the following magic items...
- robe of the archmagi
- two spell scrolls of find greater steed
- wand of polymorph
- eight tomes of clear thought
- ring of spell storing
(a headband of intellect is a suitable, and more reasonable replacement to the tomes of clear thought)
... and a large enough party to keep Niv-Mizzet busy (including sufficient spellcasters to have ample counterspells and be able to get through a prismatic wall if Niv-Mizzet decides to enclose himself in one)
The first step is to get yourself a loyal Intellect Devourer. This is actually extremely easy.
- Use the first spell scroll of find greater steed
- Cast true polymorph turning the steed into an Intellect Devourer
- Wait until true polymorph becomes permanent.
Then you need to make your intellect devourer a genius.
- Allow your intellect devourer to occupy a host that has the capacity to study a tome of clear thought
- Have your intellect devourer study until its Intelligence is 30 (you should also apply as many tomes of leadership and influence and of understanding as you have in order to bolster its mental ability scores)
- Have your intellect devourer leave that host body, so it is ready to infiltrate Niv-Mizzet.
Finally, cast sleep, feather fall, and enhance ability into your ring of spell storing so you can use it later even if you run out of spell slots during the fight.
The fight will take quite some time until Niv-Mizzet will succumb to our strategy. As such, every party member will need to be able to chase Niv-Mizzet when he catches on to the strategy (and he most certainly will as one of the foremost geniuses in the D&D multiverse), and you will need ample clerics to keep you alive as you will become the target. The gist of the strategy is to land a polymorph on Niv-Mizzet to make the rest of the procedure easier.
- Cast bestow curse (disadvantage on wisdom saving throws)
- You should also have your party attempt this too where possible, including the bane spell. This is especially helpful since your party members have space for sorcerer levels for the Heightened Spell Metamagic.
- Cast polymorph until it works
- This could take a while, but your party can also help. Once Niv-Mizzet recognizes your strategy, he will be trying to counterspell these castings when possible (something he can theoretically do indefinitely because of his Dracogenius Legendary Action).
- Eventually you will get through his Legendary Resistances and Magic Resistance to turn him into a Giant Centipede (the CR 1/4 beast of lowest Wisdom)
- Have feather fall at the ready in case Niv-Mizzet decides to try to end polymorph via the bludgeoning damage from falling.
- Have a party member cast their own polymorph and end yours (if one of your party members wasn't the one that was successful in the first place)
- This opens your concentration for true polymorph
- Cast true polymorph on the Giant Centipede, turning it into a Bullywug (the CR 1/4 humanoid of lowest Intelligence)
- your party member can now end his Concentration on polymorph
- Put the bullywug to sleep
- Use your second spell scroll of find greater steed to resummon your intellect devourer
- Cast enhance ability (Fox's Cunning) and use Bardic Inspiration on your intellect devourer
- have one of your party members cast guidance as well
- Have your intellect devourer use Body Thief on the bullywug
- The will succeed 99.9% of the time (as shown in this anydice function)
- Your intellect devourer now carries Niv-Mizzet's statistics (as a dracolich), albeit in a true polymorphed state.
- Finally, simply end your Concentration on true polymorph and you now have your permanent undead follower: Niv-Mizzet the Dracolich
- Because your intellect devourer now has counterspell and 9th level spell slots, you can just ensure that you counterspell any attempt to cast wish to restore Niv-Mizzet's brain and attempts to cast protection from good and evil to remove your intellect devourer.