I want my players to get to level 20 before fighting Karzoug; I have some ideas on buffing Karzoug, but I need some help fleshing out encounters to get the team from level 17 to level 20

[KoolKobold]

So the IRL campaign is going well, we're starting Book 5 in an interesting light, giving some character development with our paladin, utilizing some of the Demon Knight NPCs from the Villain Codex. And I got an idea:

Right before fighting Karzoug, the room before they get to the Eye of Avarice seems like a good place to rest. But Lamashtu-who has been crossed multiple times by the PCs-decides she'll aid the Runelord of Greed by personally handling them. This doesn't mean fighting them directly, but since the Mother of Monsters is also known for nightmares, she would create a sort of prison for the PCs that would involve 3 mini dungeons that each appear to be warped copies of dungeons they previously explored.

Dungeon 1-or the Catacombs of Lament as I call them-looks like a mix between the Catacombs of Wrath and Thistletop's lower dungeon. There, they will fight various half fiend goblin fighters, an incubus brawler that resembles Tsuto, a greater barghest arcanist (meant to be Malfeshnekor but freed thanks to Lamashtu), and finally, the Lost Warrior-a succubus warpriest of Lamashtu that heavily resembles Nualia. Defeating the Lost Warrior frees them from the Catacombs and gets them to level 18.

Dungeon 2-the Mansion of Murders-resembles the Foxglove Manor, only with walls and floors made out of corpses. Multiple variants of corpse orgies, who are composed of various ghouls and having ability drain, prowl the areas, with most of the threats being high level haunts that deal devastating effects. While they PCs try to find the heart of this dungeon, they end up taking random attacks from something unseen. It is when they go to the heart that they are attacked by another, larger ghoul orgy, and the main boss of this dungeon-a babau slayer named the Obsessed Lord that heavily resembles Aldern Foxglove as a ghast, but now flayed and with burning red eyes. Defeating him frees the PCs from the Mansion and gets them to level 19.

The final dungeon resembles Fort Rannerick, nicknamed the Slaughter Grounds; gigantic in scale, the common enemies here are half fiend ogre barbarians, with the spirit of Mokmurian being a creature unable to be harmed directly, but can be hit by magic-as he constantly casts spells to wear the PCs down. The final boss here is a half balor ogre barbarian (or bloodrager) called the Carnage King that resembles Jargraath, now with the skull ornament of a balor and a giant greataxe made of flame. Defeating him frees the PCs from Lamashtu's trap and brings them to level 20.

For Karzoug and his minions, I'm thinking the best way to buff Karzoug is to give him a few mythic tiers in archmage-this makes a lot of sense as every single other runelord that has stats, to my knowledge, have a tier or two in that mythic class. The dragon and the rune giant can be given the advanced templates, and if that isn't enough, a class template too.

All of this is...wild. I'll admit it. But the PCs are looking forward to kicking ass and taking names at level 20. And I'm having so much fun that I want to give them something that they will always remember. Plus, Lamashtu has been a constant theme and I've inadvertently given the feeling that she's been watching the PCs in anger at their endeavors, so it would feel fitting for her to try and screw them over right as they're facing the BBG.

Any advice will be more than welcoming. I can get the stats down myself, but more suggestions on how to build up these new encounters, and buff up the final encounter for the grand battle.

[Bjørn Røyrvik]

One of the easiest solutions is to just give more xp in the remaining encounters. Most players won't complain.

As for how to run Karzoug: I too increased him to 20th level. I didn't bother with Mythic rules per se but I did give him a quickened spell each round and a bunch of other free metamagic and extra high level spells. Most importantly, I made sure his spell list was primarily Transmutation spells; he is a master of Transmutation, after all. Many spells were from previous editions of the game, which was a nice hint that he knew forgotten or unique spells unknown to the PCs (often the players).

The Eye was honeycombed with small tunnels with peepholes so he could stay in Sand Form and rush around the Eye and snipe the PCs with spells and be almost impossible to target in return.

Some of the spells I used:

Disintegrate Flesh
(Transmutation)
Level: Sor/Wiz 9, witch 9
Casting Time: 1 standard action
Components: V, S, M
Range: long
Duration: Instantaneous
Area: 40' cube
Saving throw: Fortitude negates
Spell resistance: yes

You disintegrate all organic matter in the area. Inanimate or non-living organic matter is destroyed without a save, though magic items receive a save, as do things attended by a person, like worn clothes. Creatures that fail the saving throw take 2d6/level points of disintegration damage.
Material Components: a tuning fork of cold iron

Sand Form
(Transmutation) [polymorph]
Level: Sor/Wiz 9
Casting Time: 1 standard action
Components: V, S, M
Range: personal
Duration: 1 day/level (D)
Area: a body of sand

You create a body from sand that houses your mind similar to an astral body. The sand body has the same volume as you, but can spread out or clump together as you wish so long as the mass is contiguous. You move at your normal movement speed, and are still in full possession of your normal senses while using the sand form but your real body is paralyzed while the spell is in effect. You need not eat or drink for the duration. Your sand body suffers a -4 penalty on any roll requiring fine dexterity (e.g. disable device or playing instruments) but otherwise has your ability scores, saves, natural AC, etc. and can do anything you can. While in sand form you are immune to poison, physical ability damage or drain, precision damage and, like Dust Form, count as incorporeal and can shape your body . If exposed to heavy rain or submerged for ten rounds or more, the sand body crumbles. Otherwise the sand body has the same number of hit points. The sand body can only function within 100 miles of your original body. You can cast spells through the Sand Form, but must have the necessary components with it.
Material Components: a fistful of sand and some hair or skin from the caster.

Solidify Air
(Transmutation) [air]
Level: Sor/Wiz 9
Casting Time: 1 round
Components: V, S, M
Range: medium
Duration: 1 round/3 levels
Area: 10' cube
Saving throw: Reflex partial
Spell resistance: yes

You cause the air around the target to become as hard and unyielding as stone, trapping them inside. On a failed saving throw, the target is also incapable of speech and breath, and begins to suffocate. On a successful save the target is still immobilized but can breathe, speak and make minor movements of extremities, though not enough to permit somatic components for spellcasting. The solidified air prevents any physical or energy damage from going into or through the area, though light effects are not hindered. Creatures larger than the area of effect can be partially trapped by it, and the spell will prevent the movement and effect of many effects, such as harmful gases and clouds. Creatures can walk on the surface of the area without trouble, and it can function as a short-lived wall, floor, roof support or similar. Creatures of elemental air are not held by the spell but merely slowed for the duration on a failed save and slowed for only one round on a successful save
Material Components: a cube-shaped diamond worth 1000 gp

Spell-lash
(Transmutation)
Level: Sor/Wiz 9
Casting Time: 1 immediate action
Components: V, S
Range: personal
Duration: Instantaneous
Area: 25' radius
Saving throw: Reflex half
Spell resistance: yes

This spell completely destroys your body and deals 25d10 points of damage to all creatures and items in the area.
[I know this seems like it should be an Evocation spell, but it was originally listed as Alteration, so I kept it that way]

[KoolKobold]

I do level up by milestones, not xp. And I want to have the PCs feel like they earn it by duping Lamashtu’s trap.

I will definitely be considering using those spells, and will have Karzoug act with two spells (one quickened) to show how strong he is.