here's some epic level monsters and powers i've used

How to challenge high level characters, piloted by players I've been playing with for a decade who know me and my tactics intimately? By level 20, they were handily taking down monsters with 500+ hitpoints and extravagant amounts of armor class and damage potential. In the Temple of Demogorgon, I put in a hechatonchieres which was meant to be too scary for them to fight. They focused fire and took it down without a casualty.

At the same time I've been playing a lot of Slay the Spire this year. The game has a lot of great monster mechanics that play with your powers in really interesting ways, and makes even normal combats scary enough to kill you under the right circumstances. So many good powers which really fuck with your tactics and interact in so many interesting ways with your cards and powers, allowing an infinity of tactics, and an infinity of ways to fuck up. So I made a list of similar powers of my own. Most of these did not allow saving throws or other soft mitigation, and instead became hard boundaries that players were forced to navigate.

1. Gain Strength each round -- damage increases linearly at the start of each monster turn. Even a weak monster can quickly become a serious threat.

2. Gain an extra attack each round -- very similar to the above, though it interacts differently with player abilities.

3. Gain an extra attack for each action taken by an enemy in its vicinity on the preceding round -- players who shrug and hope for the best quickly pay the price

4. Deal additional damage as the target's hitpoints decrease

5. When it deals damage, it is attached to you and moves with you. If you detach from the monster by any means, take additional damage

6. When an attacker deals damage to the monster, the attacker takes retribution damage 

7. For every two points of damage dealt the target, each ally takes 1 psychic damage -- a really rough deterrent that interacts in interesting ways with persistent hazards

8. When the monster deals damage, an equal amount is dealt to the character with the lowest hitpoints -- a very rude power

9. Every time a magic item is used near the creature, it gains hitpoints

10. When the monster deals damage, the target loses a spell slot of a random level

11. When the monster deals damage, it dispels a magical effect on the target

12. Every time a spell is cast near the creature, another spell slot of a lower level must be expended

13. Whenever an attacker misses the target by a certain amount, it can counter-attack for free 

14. When an attacker kills a monster, that attacker's next source of damage is reduced to 0

15. At half health, the monster splits into two monsters of equal health

16. The monster ignores the first instance of damage dealt to it each round

17. AOE effect causes targets to deal half damage and move at half speed

18. Immune to all magic and magical weaponry

19. Turn incorporeal and teleport long distances for free

20. High regeneration

Some examples of stat-blocks for monsters they've killed:

GUARDIAN APE

Giant white ape 30 feet tall, stained with blood down his mouth and chest, with bones littering the ground around him. He's sleeping peacefully and makes perception checks at disadvantage until woken. 

HP 600, AC 24, Move 90, and an additional 90 as a bonus action. He can jump, bound, and leap the full 90 feet.

Magic absorption: Any spell cast in the ape's presence causes the caster to lose another spell-slot of a lower level of the target's choice.

Berserk: The ape attacks with advantage, and all attacks against it are made with advantage. At the end of his turn he gains a +4 bonus to damage with all attacks. This stacks and lasts for ten minutes.

When reduced to 300 HP, divine light rains from the sky. First story ceilings collapse, buildings are ruined.  All magic effects on the ape are dispelled. All enemies have one magic effect removed from them. Also, all enemies must make a Con save DC 24 or take 6d6 radiant damage. Furthermore, the guardian ape is surrounded with divine light, which persists until the end of the player's NEXT turn. For every 2 points of damage dealt it, every ally of that character within 300' takes 1 psychic damage, even if they are out of line of effect. After this effect ceases, the rain will repeat at the beginning of every other turn.

ATTACKS:

3 punches +16 for 7d8+16 damage. This can punch through walls and buildings.

Grab, bite, and throw +16 for 21d8+54 damage and get tossed up to 100' in a direction of the ape's choosing (off a cliff)

2 CRIMSON SCORPIONS 

They are the size of mack trucks and sing an anarchic, disharmonious, wordless duet.

HP 300, AC 26, Move 40

Each time a scorpion is dealt damage, its attacker takes 2d8 psychic damage

It makes Strength based checks at +20

Attacks:

Claw/Claw/Tail +16 : 4d12+10 and grab/4d12+10 and grab/2d8+10 and Con DC 23 or 10d10 poison damage and forget 1d4 prepared spells, chosen at random

Spray bubbling green poison in a 90' cone (Recharge 5/6): Dex DC 23 or 14d8 poison damage and have all magical effects on you dispelled, on success take half damage and no dispel

KATANA-WIELDING MONKEYS

There are 3d20 of them.

HP 45, AC 17, Move 60

2 attacks +12 for 1d8+6 damage

They may use an action to pray for strength, gaining a stacking +4 damage bonus that lasts for ten minutes. 

SER YHARNAM THE RED

A knight in armor made of tempered cardinal feathers, wielding a greatsword made of bluejay and blackbird wings, and wielding a greatshield emblazoned with a mouse and seven stars. His longbow has a bright red bowstring.

HP 300 AC 26, or 30 when using his shield

Undead and no longer sentient, but implacable and fights intelligently

All saves vs magic are made with +18

Every time a magic item is used within 300' of him, he heals 100 hitpoints (no maximum)

If an attacker misses him by 5 or more, he may immediately counter-attack. He can do this any number of times per round, but he can't counter melee attacks with his bow, or long-ranged attacks with his sword.

Attacks 4 times per round with greatsword or longbow: +15 2d8+10

When he hits a target with his longbow, they must make an Int DC 23 saving throw or lose a random spell slot

FLAGELLANT MONKS

Dead monks hanging from the walls and ceilings, they carry whips with broken stone shards tied to them, their robes hang in tatters, the skin on their backs is hanging off them in bloody strips and the congealed blood which runs out their backs is sticking to the walls and ceilings like a slime and sending down tendrils to puppet their heads and hands like sick marionettes.

5 of them

HP 120 AC 21

Move 30, can walk on walls and ceilings

Undead

When reduced to 60 or fewer hitpoints, they fall from the blood slime that holds them, and are no longer able to walk on walls and ceilings. The blood slime becomes an independent monster with as many hitpoints as the flagellant monk was reduced to. i.e. If an attack reduces a monk to 61 or above to 60 hitpoints, a 60 hp blood slime spawns. If an attack reduces a monk from 61 or above to 1 hitpoint, a 1 hp blood slime spawns.

Flagellant whip +16, 3d8+10 and take +5 damage from subsequent attacks from flagellant whips 

or spit congealed blood up to 60': All in a 15' radius must make a Dex save DC 24 or be slowed as the spell, and may continue to save at the end of each turn. Monks and blood slimes are immune to this.

BLOOD SLIME

HP = whatever hitpoints the flagellant monk was reduced to. AC 21

Move 30, can walk on walls and ceilings

When a blood slime is killed, it explodes congealed blood every, slowing targets in a 15' radius just like the blood spit above. Monks and blood slimes are immune to this.

Bludgeon +10, 8d8+15 damage 

what have we been up to? (2021 round-up)

 Well that's a very big question!

They were in Hell for a long time, in the Temple of Demogorgon. They went all through it and completed a long puzzle in the depths of the temple that let them speak with the Archbishop Gorgrudoch. They killed him for the Crown of Golden Death and rode the Golden Throne in the Nave of Hethradiah down to the Secret Cathedral of Demogorgon and fought the Archbishop Aggreal (there are two archbishops, just like Demogorgon has two heads. Demogorgon keeps his identity secret, his name is forbidden -- Sad Ed the Bard made the mistake once of uttering his name inside his temple, and He responded by striking Sad Ed blind and deaf and summoning a swarm of nalfeshnee to tear them apart. Each nalfeshnee is able to summon another nalfeshnee. This led to a very long, chaotic battle as my players realized that the nalfeshnee chain of summons could be infinitely long, and to this day they refer to Demogorgon as "The Sinister Scorpion" to protect themselves from his ire), and after reaching for the Skoros Orb in the black vault behind the statue of Demogorgon, discovered that the corrupt demonic flesh of the colossus the temple was carved into had grown around the orb and created a Skoros Orb Beholder. So then they fought a deranged and gigantic beholder which shot swords at them and froze them in ice blocks and made them love it and oriented their gravity away from it and manifested demons and shot disintegration lasers and sprayed blue slime and gamma rays. Then after they killed it they threw the corpse of the Archbishop Aggreal into the pit where the skeletons of all the past archbishops were kept, and departed hell with the Hell Soul of the Skoros Orb.

Then they tracked their nemeses, the Deadboyz, to a little town in the Goblin Wastes called Belroun. The Deadboyz had the remaining piece of the Skoros Orb. This was a dusty western town with tumbleweeds and a brothel and a saloon and a witch and a ghost seller and an old church to Nito I mean Otin the Gravelord, underneath the church was catacombs where was interred all the dead who fell in Belroun's rebellion against the monarchy of Orostranthy. This resulted in a long skeleton-themed dungeon crawl that was semi-procedural, well at least semi-random, where they fought eye-bats and bone golems and fought a gigantic skeleton in a pit of green slime as well as an enchanted suit of armor, which they killed by disintegrating the stairs under it and causing it to topple into the slime, and also at the same time coincidentally revealing the entrance into the Deadboyz' secret hideout. (This was actually a coincidence, they did actually on accident and completely coincidentally target with disintegrate the exact square in the entire dungeon that hid the entrance to the Deadboyz' hideout).

So then there was a long stand-off and slow battle with Blank the Wizard and Axen Great the Thief. My favorite part was, after the dust had settled and Blank had been killed and then reincarnated himself in the body of the soulless child he kept tied to his belt at all times and teleported away to safety somewhere, and Axen Great was trapped in a force cage, and the players had dispatched the time-eating fire elemental that gained an additional attack each round for every action used in its vicinity, the players decided to investigate the obviously trap-filled hallway packed with furniture just outside the room where they did battle. I don't remember why they did this, they knew it was definitely trapped. But each end of this hallway had a combination tumbler lock, so they figured something fucked up would happen if they forced the lock, and decided to carefully pick it instead. They were correct, but unfortunately for them the secret was that the locks were decoys and the door had a secret password that had to be spoken aloud ("There's a great treasure around the corner"), so even picking them caused both doors to close and lock and fire to spray from the ceiling, igniting the furniture that was packed with gunpowder, thereby causing the hallway to explode and collapse. Dirtface the Barbarian did some quick thinking and took shelter in his extradimensional pocket dimension, and we had to figure out on the fly whether force from outside could affect the pocket dimension after the door was closed. Anyway he survived, and they looted the third piece of the Skoros Orb from Blank's room, where it was protected by a death rune that constantly radiates 100 damage that ticks twice per round. I think they used an antimagic field.

Then they got nervous about combining the three pieces of the Skoros Orb, because in the course of their adventure they figured out that it got its mighty power by summoning the ancient SKYCANCER and trapping it in the orb and concentrating its power into a single point. The problem being that the Skycancer's influence seems to corrupt the orb's mightiest power, a single Wish granted to the holder. So they went to some libraries and looked up old pictures and accounts of it, learning that in the old days it was a horrible demon the size of a storm that flapped around and ate entire cities at once. They talked to the Purple Dragon Terrifex, who once battled the Skycancer in the freezing skies over the arctic Kraal, and did not win. And then they spoke to the time-stopped wizard Yote, who helped create the Skoros Orb 2000 years past, and tricked him into giving them the Rite of Purification, which was meant to force the Skycancer out of the orb and purify its use. "Would that, like, would the Skycancer then be like, free and stuff?" "Yeah!" said Yote. They set him on fire and put him back in his stasis.

Satisfied that the orb wouldn't immediately kill them after recombining it, they proceeded to the Seat of the Skoros Orb, a pearlescent structure of coralescent magic in the depths of the Shattered Labyrinths of Illith Varn, and there placed the three orb pieces, which combined into the true Skoros Orb (Corrupted), which nevertheless has a ton of mighty powers. Among them, the ability to conjure magical fogs, maintain concentration on spells even if the caster falls unconscious, decrease the casting time of all spells to a single bonus action, not to mention the nine unique spells of the Skoros Orb, and a +3 bonus to attacks and difficulty checks... Jaime the Wizard immediately went mad with power and has not let hold of it since.

But they found themselves locked in the depths of the labyrinths, unable to teleport out until the orb witnessed the sky a final time. So once again they made the long trek up, through the Temple of Aameul, through the flooded swamps of the Demon of Song, and into the scorched ex-headquarters of the wizard Malagon of the Cruelest Eye, and there found themselves ambushed by Malagon himself, and his two compatriots, Kaviel the Strange and Ser Senedar the White, once the three generals of the dead Illith Varn, and they demanded the party hand over the Skoros Orb. Sad Ed the Bard made a good effort to break their morale but failed his final Charisma check, and the party was forced to do battle with all three generals at once in the heart of Malagon's headquarters. So this was awesome. Malagon was creating disintegration webs and Kaviel was wielding his massive uchigatana and casting Fire 3 and Thunder 3 and Senedar, an aged warrior wielding a white katana, was carving everyone up. But ultimately, the team was able to mind control Kaviel into eviscerating Malagon, and killed Senedar where he kneeled, and victorious, made their way to the surface, where they found their camp under attack by The Sky Witch Queen Jupiter Volaris, atop her mount, the Green Dragon Corvenon, Son of Valathex.

Another boss battle. This is basically the boss rush section of the megadungeon, okay? Anyone they didn't deal with earlier comes back to steal the Skoros Orb from them. So they go rushing into camp and the Sky Witch is conjuring lightning from the sky and Corvenon is breathing gas that's transporting them into the world of dreams and the Sky Witch is putting curses on them and camp is a mess and their allies are hiding in tents and the woods and stuff, and the knight Vlawyn the Ket is pinned to the ground by a wind spell, and there's a mighty storm and pouring rain that's keeping them from moving at full speed, and the Sky Witch casts a spell that makes Malachai the Dwarf shoot straight up into the sky at a rate of 400' a round, but the team has a good idea, and Sad Ed gets up next to the Sky Witch and uses his magic crimson ring that causes him to switch places with the wearer of its twin crimson ring, luckily worn by Malachai the Dwarf, who then decapitates the Sky Witch and slaughters Corvenon. The storm ends, the dust settles, they bring their dead compatriots back to the life, and the party realizes that the Sky Witch left behind her Flying Fortress.

"Just a quick look around to make sure it's safe. We cleared pretty much everything the first time through, but I think we missed a couple rooms"

They go inside and immediately fight a Titanite Demon in the room they missed. They find the World Map in the witch's bedroom, with a mic that connects straight the Saliflax the Fire Elemental who drives the fortress. They discover that the fortress can't land, as it's been cleaved from the earth until the end of time, and if ever it does touch land, it will never be able to fly again. They break the news to Saliflax that they killed his boss, which he's sad about, but he gets over it quick because he's a Fire Elemental and they work through their feelings faster. And then they come across a gigantic painting in the ballroom of a stone building on top of a snowy mountain. They never got the chance to fuck with it before, since previously this room was the site of a masquerade put on by wind spirits, who had all been killed by another, more evil, more powerful spirit the players accidentally/on purpose released. Anyway Dirtface the Barbarian, who's played as much Dark Souls as me, gets excited and goes up to it. It's like thirty feet tall.

"Does it draw me in?"

"No... SMH"

"Is there anything on its frame or like underneath it?"

"No? There's nothing there"

"Hmm... I touch the picture"

"Oh, it tries to draw you in"

Very excited, the whole team gets drawn into... THE PAINTED MONASTERY OF ST LLOYD THE MONSTER.

Cue, a mixture of Aramis, Ariandel, Senpou Temple, and Sunken Valley. A frozen monastery on a mountain-top guarded by katana wielding monkeys and two crimson scorpions singing an anarchic, unharmonious song. In the depths of the monastery, the former monks, corrupted into dragon-like abominations by the blood they had injected into themselves, which they had harvested from St Lloyd himself, now locked away in the monastery's inner sanctum. Mosquito men, mimics, alligator men, undead monks controlled puppet-like by the blood they had shed in self flagellation. A sane and uncorrupted monk, who murdered his abominable brothers, and locked himself in a cell in penitence. A thirty foot guardian ape protected by divine radiance. A long-bearded monkey master and his family of golden monkeys. A corrupting disease settling into the bodies of all who dwelt here. A headless, rotten undead dragon, sitting on a snow-covered bridge. Talk of the original monastery, a Monastery of the Black Wind, where this painting was made to hold the abominable Saint Lloyd, who is said to have come across a wounded Divine Being in the wastes, and mingled its blood with his own, thus becoming Sainted. They were told that Saint Lloyd guards the only way out of the painting.

Me: "You could also cast Gate or Plane Shift to leave any time you want."

Them: "That's Okay"

Proceeds to teleport the undead dragon off its bridge and cast Meteor Swarm on it from afar as it flails helplessly in the snow 600' below until it dies.

They're level 23. We're having fun!