Stage one: Instead of using actual wizards, use perfect autonomous illusions of yourself. Does this require reverse-engineering the actual Thassilonian spell from the decayed version practiced in modern Golarion? Yes. Will you blow yourself up doing this several times? Yes. Will you blow up these tremendously expensive perfect autonomous illusions that cannot be resurrected or (except at great expense) repaired? Well, occasionally.

Stage two: The country can take a little magical spillover, right? Horrifying crop blights, new mutant plagues, trees eating people, these are just hazards of life in the post-apocalypse, right? Nobody will even notice you're doing it. Right, that's what you thought. See, all these "secondary balancing circles" are completely superfluous, just perfectionism from an ultra-rich society.

... Okay, most of them are. The quinary ones, definitely. Sure.

Stage three: Once you have enough ritualists (see: copies of yourself), hack together a ritual (see: blow yourself up several times) to instill complete confidence that YOU ARE A GOD and absolute obedience in any wizard who already has the vague generic belief that you might sort of be a god, as well as terrifying divine powers from shards of dead gods or whatever you found lying around. Then, leverage this to get more wizards. Repeat until you have a sufficient ritual magic team to carry out a ritual to turn a preexisting cult into Actual Ascension, by conquering the planet if necessary.

... Actually, no, that's step four. Step three is building a fortress.

Okay, step three might be conquering a country - 

- At any rate. Several times a year, Razmir carries out the ritual, in person, summoning his simulacra from his crafting-demiplane in the robes and masks of ordinary priests, to transform the new graduates of his brutal project of brainwashing into a mixture of corpses (who he, obviously, reanimates as undead so they can tell no one what occurred) and faithful priests absolutely loyal to him. (He uses drugs, not complicated spells, to induce memory loss of the actual night of the ritual; it's fine if it's hazy, as long as nobody can reverse-engineer what he's doing from it.) He does this in First Step, a fortress he constructed at the heart of the Exalted Wood, which used to be a perfectly ordinary forest and is now a blighted horror.

In person.

On the Material Plane.

At predictable intervals.

No, he has spared no expense making First Step invincible, and making not merely the central structure but everything within a very long distance of it disintegrate non-Lawful Evil people who don't know the password. (Passwords. Different sections of the complex have different Forbiddances, obviously.) Yes, there are guards. Yes, there are lots of guards. Yes, there are very scary guards. Yes, whenever he visits he casts all of his hour-per-caster-level buffs the day before and prepares combat spells for the trip. Razmir isn't stupid.

... Really? You sure about that?

I am, why.

- Because around the time Razmir began his ritual, Remek Czaszar, in his dark fortress read a scroll of Wish penned by the hand of the Whispering Tyrant himself to steal Razmir's Clone out of its demiplane and teleport it to an extremely secure location, where it was stripped of its magic items and many dark and powerful curses were cast -

HAH! Fool of an Avistani savage! Primitive mortal who scrapes and begs at the feet of ignorant shamans striking together flint and steel in the arrogant belief that this makes them gods! Child of barbarians, born to pain and doomed to death!

Why in the name of ascendant Lissala would I only have one Clone?

Razmir will continue his ritual with no particular worry, since his odds of waking up in the one clone the undead stole are negligible, even in the spectacularly unlikely event he dies. He has ways to track it later, even if they've warded their fortress against scrying, and then he can turn this aforementioned fortress into a direct pit leading to the Abyss until he gets bored.

Unfortunately for Remek Czaszar, this exchange is not, actually, taking place in a world where he can learn from it, and, convinced of the success of his master plan, he will initiate the second stage of it on schedule - 

- When the ghost of Julius Fabrius Vindex moves the hands of Viktor Sovin to cast a Greater Dispel Magic that Sovin could not possibly have had the sleep to prepare, targeting the Forbiddance around the ritual location.

And that is when the undead armies all Teleport or Greater Teleport to, approximately, Right On Top Of Razmir, give or take some Teleport-based inaccuracy.

A brief digression about the nature of undead armies in Ustalav:

Some might ask, "why, if you can just turn everyone you kill into undead minions, does this not lead to an endless chain reaction where you kill people, who become your undead slaves and go on to kill more people?" Surely even if you need to cast individual Animate Dead or Create Undead spells, a thousand-year-old lich could have basements full of hundreds of thousands of skeletons, waiting patiently to be unleashed?

The answer is simple and brutal: Unless you are the Whispering Tyrant, who just Breaks All The Rules, even the most powerful undead can only control a finite number of lesser servants without them breaking free, scaling with the strength of the necromancer's life force. A vampire can control two, or three, or maybe even five spawn, if they do are not powerful spawn; the mightiest extant, slumbering Malyas, might be able to manage eight. Undead can control those they create, up to a limit; necromancers can control those they raise, up to a limit, and dark priests or necromancers can temporarily (and, with the right curses, less temporarily) enslave any undead whose will they can conquer. Up to a limit.

Separate limits, of course. A great lich such as Remek Czaszar might have seventy or so skeletons and three potent vampire wizards under him, each commanding forty skeletons and three spectres who each commanded two lesser spectres, two lesser vampires who commanded together five spawn and six shadows and another eighty zombies.

Call it necrofeudalism.

Remek Czaszar has a great innovation on this that places him ahead of his rivals, which is having Lawful minions sworn to loyally and obediently serve you, so you don't need to devote valuable resources to controlling them by magic. But, fundamentally, necrofeudalism is still the basis of his army.

The RIDICULOUSLY HUGE UNDEAD ARMY that teleports in therefore mostly consists of a handful of genuinely ridiculously powerful wizards who provided the teleports (all, of course, covered with enchantments to make them harder to kill) with also-scary flunkies carrying Bags of Holding that they pour out on the ground, from which vast armies of skeletons, spectres, wraiths, shadows, vampire spawn and so forth emerge.

This is, legitimately, a way of attacking with more people than you can manage in one teleport.

It also takes more than a round to kill people.

And then Razmir gets a Sending.

Riudaure dead. Need a Wish to resurrect him before Hell finds out about Iomedae's backing of your ascension.

RAZMIR IS BUSY, HERE 

What he actually says is Tell Morgethai to do it, while simultaneously hurling the ritual wand that is the focus of the ritual aside (The ecological consequences on the Exalted Wood are not to be guessed, but the sheer forces vented through the ritual result in the wand exploding, forty-foot-tall gusts of fire jetting out where the other three top ritualists stood, and Razmir himself being harmlessly incinerated) and flying backwards forty feet, his cloak autonomously deflecting attacks from the attackers who surround him as he orders all of his simulacra to teleport to safety and all of his other minions to DESTROY THE INTRUDERS -

(He has a really good headband.)

- He will then snap his fingers, causing a Rod of Greater Extend Spell to drop into his hands, and attempt to cast his prepared Time Stop, because actually screw these people.

Three different seventh- and eighth- circle wizards attempt to counterspell him with Greater Dispel Magic! One of them succeeds.

- Snaps his fingers again and the Rod of Greater Extend Spell is replaced by a Rod of Quicken Spell and go for his Plane Shift -

You really have no idea how many people you ticked off, do you?

(At this point, an epic battle is breaking out between undead - spectres, wraiths, shadows, wight captains, mummies both traditional and Osirian, hueceva clerics, vampires, and all the other vast hordes that necromancers can bring to bear - on the one hand and the most trusted guards of Razmir - kytons, inevitables, golems, and his false priests - on the other, which is far too complicated to do more than summarize here. Razmir has the advantage of total weight of forces, but the disadvantage that most of his ritualists just exploded.)

Extended Time Stop, also, SCREW YOU!

Greater Dispel Magic, comes back the chorus, and also magic arrows.

Quickened Plane Shift!!!