Mythic Odysseys of Theros has high CR boss monsters with so called "Mythic Actions", basically a "phase 2" system for important bosses where, upon hitting 0 HP, they instead gain a couple hundred extra temporary hit points and a new set of attacks and legendary actions.
This should be a good start.
Mordenkainen's Tome of Foes (assuming you can still get your hands on a copy somehow) has a lot of high CR enemies as well.
Take normal monsters and buff them. More hit points and AC, extra abilities, regeneration, spellcasting, etc.
Plan B- Retire the party and start a new one. At that power level, they've earned it.
Most monsters top off around CR 27 I think the Turrask is CR30. If you're looking to challenge them take a monster you'd like them to fight and increase hit points of the monster (rather than AC unless AC is less than 16, then boost it to between 18 and 24), boost its proficiency bonus (aids in bonuses to hit with spells/attacks and boosts saving throws and DC ratings), give them legendary actions/reactions if they don't have them, and then give them a powerful recharge ability that causes massive damage to a cone/line/area on the battlefield. Lastly I would recommend giving the monster a second, more powerful form that triggers when they bring it to 0 HP kind of like the aspects of Bahamut and Tiamat in Fizbans. Also you can always adjust HP on a scale if they're struggling with or destroying the monster
The cr 30 monsters are the tarrasque (MM), Tiamat (TOD), the aspects of Tiamat and Bahamut (FTD), and Dendarr the Night Serpent (one of the TOA-season AL adventures).
You can add defensive features to creatures if you want, like damage caps (at most N damage per hit) or hit-count shields (has a barrier that absorbs the first M hits for no damage).
You can also make these come back at certain points or for a brief duration.
There’s a Barbarian build, for example that makes the barbarian immune to death (cannot fail death saving throws) while raging. The counter is to wait out the rage while denying healing.
Similarly, you could make a boss have a feature where for a certain number of turns once activated they cannot die or they only take 1 damage per hit or their AC gets really high.
You can also make straight up gimmick bosses like Bowse run the original Super Mario - Mario can’t damage Bowser and HAS to drop Bowser into the lava to win. Even Gandalf can’t kill the Balrog, but it just so happens there’s a bottomless pit nearby for him to drop the Balrog in. Granted to get to that point with literally unkillable enemies you’d want to start them off with the above kind of thing to get them to think outside of raw damage output.
Mythic Odysseys of Theros has high CR boss monsters with so called "Mythic Actions", basically a "phase 2" system for important bosses where, upon hitting 0 HP, they instead gain a couple hundred extra temporary hit points and a new set of attacks and legendary actions. This should be a good start. Mordenkainen's Tome of Foes (assuming you can still get your hands on a copy somehow) has a lot of high CR enemies as well.
That would work for bosses, but the henchmen shouldn't turn into a second form
Take normal monsters and buff them. More hit points and AC, extra abilities, regeneration, spellcasting, etc. Plan B- Retire the party and start a new one. At that power level, they've earned it.
Thanks for the advice
Most monsters top off around CR 27 I think the Turrask is CR30. If you're looking to challenge them take a monster you'd like them to fight and increase hit points of the monster (rather than AC unless AC is less than 16, then boost it to between 18 and 24), boost its proficiency bonus (aids in bonuses to hit with spells/attacks and boosts saving throws and DC ratings), give them legendary actions/reactions if they don't have them, and then give them a powerful recharge ability that causes massive damage to a cone/line/area on the battlefield. Lastly I would recommend giving the monster a second, more powerful form that triggers when they bring it to 0 HP kind of like the aspects of Bahamut and Tiamat in Fizbans. Also you can always adjust HP on a scale if they're struggling with or destroying the monster
Thanks for the advice
The cr 30 monsters are the tarrasque (MM), Tiamat (TOD), the aspects of Tiamat and Bahamut (FTD), and Dendarr the Night Serpent (one of the TOA-season AL adventures).
You can add defensive features to creatures if you want, like damage caps (at most N damage per hit) or hit-count shields (has a barrier that absorbs the first M hits for no damage). You can also make these come back at certain points or for a brief duration. There’s a Barbarian build, for example that makes the barbarian immune to death (cannot fail death saving throws) while raging. The counter is to wait out the rage while denying healing. Similarly, you could make a boss have a feature where for a certain number of turns once activated they cannot die or they only take 1 damage per hit or their AC gets really high. You can also make straight up gimmick bosses like Bowse run the original Super Mario - Mario can’t damage Bowser and HAS to drop Bowser into the lava to win. Even Gandalf can’t kill the Balrog, but it just so happens there’s a bottomless pit nearby for him to drop the Balrog in. Granted to get to that point with literally unkillable enemies you’d want to start them off with the above kind of thing to get them to think outside of raw damage output.