5E is not good at high levles. Most of the high CR monsters are weak as hell anyway.
Tome of Beasts helps a bit.
I would just ower the CR of most critters over CR 14 by 3-7.
you could also power upthings like Dragons by bringing back rules form AD&D like requiring +2 and +3 weapons,
I've been messing around with different approaches to spell resistance and immunity.spell resistance that cuase your spell to outright fail etc.
I would argue that monsters are not good for high level play - that is what we are trying to correct with this thread
I didn't find ToB to be improvement, maybe I need to take another look
As CapnZapp said, that doesn't solve the problem. Besides the CR's feel right to me, just not the actual strength of most of the monsters.
I thought that once, but I am switching back to the simplicity of magical and nonmagical. There are a lot of easy methods to make the monsters tougher.
I've been messing around with different approaches to spell resistance and immunity.
If you want to start rewriting monsters it is a lot of work. Use more monsters at higher levels or add races to NPC's.
I have a heap of custom monsters myself.
Other tweaks I have considered.
Energy drain causes exhaustion levels.
Spell resistance is a static number. A d20 roll beats it. A mind flayer with AD&D 90% MR becomes 19 or better.
Creatures requiring +2 weapons or better get resistance to bludgeoning slashing etc ncluding magical.
Spellcaster levels added to Dragons.
Magical ammunition not weapon is required to bypass resistance and immunity to magical weapons.
Otherwise its 5 minutes to tweak a creature on MS word for a basic one or half hour or more with legendary creatures.
Yes it is - that is why I have asked people to help!
Thank you for the suggestions. I takes me longer to rework a monsters (at least most of the ones I have done so far) because I feel the need to look over its history and lore and then reflect how I can add that to the 5e version without adding to much complexity. But it is work I enjoy, so there is that.
I'm not sure what the point would be. An 'advanced' DM should be able to just whip up or adjust on the fly monsters sufficiently challenging to any level of player optimization. Exhaustive PC-like design options just, IMHO, add a lot of unnecessary steps to that process.In another thread, CapnZapp suggested WotC make a monster manual for advanced players (feat using, multiclassing, optimizing type players) that are not challenged by the monsters in the MM and VGtM.
Where would legendary fit into that?Thus, I have decided to add a little trick we got from 4e: standard, elite, paragon, and mythic monsters. The elite, paragon, and mythic monsters might have the same CR, but the do more damage, have more HP and/or better defenses, and more XP than the standard (WotC) monsters. Basically to get these monsters you modifty an existing monster by 1.5x for Elite, 2x for Paragon, and 4x for Mythic.
PM me I have a personal 5E MM with custom monsters and variants. Mostly lower CR but there are a few at CR5-6. Its got 33 monsters/NPCs in it up to CR 13.
I'm not sure what the point would be. An 'advanced' DM should be able to just whip up or adjust on the fly monsters sufficiently challenging to any level of player optimization. Exhaustive PC-like design options just, IMHO, add a lot of unnecessary steps to that process.
Legendary is a different system, not a type. If you look at the monsters I have posted they have elite in the description: Large elite fiend (demon), chaotic evil Any of these could also be legendary, but they are not required to be. Notice the balor is "elite" and had legendary actions, but the marilith and goristro are only elite.Where would legendary fit into that?
Dave, I like some of what you've done here. Adding spells, some neat alternate attcks, bonus action movement, flexibility... a lot of that stuff is pretty cool.
But I think the fundamental sensibility... over-inflating damage with flat +20 modifiers, boosting HP values into 500+ and so on... I think some of that stuff is misguided, personally. It may be what you, Zapp, and others want, so more power to you. But to me it just looks like creeping power to create hard barriers.
Part of the fun of Bounded Accuracy is that there are very few hard barriers in the world. It's sad to see that eroded in a quest for Bigger Numbers.
I do love some of your abilities, though. If I wanted to use these guys, it'd be roughly as hard as using any other monster I don't like: I'd make some tweaks to get them to the power I want, and move on. I don't see why this is constantly touted as a hard to overcome hurdle.
I'm being pretty negative, so here is some positive feedback: your "Unstoppable" mechanics are a really fantastic idea, much more interesting that Legendary Resistance. I will absolutely be stealing that.

(Dungeons & Dragons)
Rulebook featuring "high magic" options, including a host of new spells.