Evauluating epic-level monster trait: doesn't fail saves

[FrancisBean]

This is a weird, random "Huh, I wonder how much of this would stick to the wall?" idea spawned by reading a few of the threads on fixing the 5e Tarrasque, but doesn't really fit into those threads.

How powerful, on an epic monster with CR over 20, would the trait: "This monster does not fail saving throws" be? I'm thinking in terms of CR computation. I know it's a little less powerful than it sounds, since intelligent parties will do enough research to know not to waste saving throw spells. My thought is that it's one of those rare situations which make martial classes really shine over the spell slingers. I just can't quantify how strong it is, particularly when it's placed on something like the CR 30 Tarrasque, and not on, oh, an otherwise CR 1/4 goblin.

Edit: Death saves are a bit of a special case, and I find myself thinking that for something like the Tarrasque it would make sense for it not to fail those, either -- that's sort of like the old regeneration it used to have. But I could see that interpreted either way, and I'm curious whether I'm missing any interactions there.

[SociopathFriend]

I would imagine a less middle-finger to any sort of saving throw would be to do something like... Spell Evasion.

All damaging spells are treated as though you can save for half even if they don't say so.
All spells with effects that last longer than one round can have a save made at the end of your turn even if they don't say so.

[DeTess]

The problem with this is that it absolutely shuts down a controller style wizard or sorcerer. I'd take SociopathFriend's solution instead for a more generic ability, but if you know none of your players ever play a controller then your idea is probably fine at your table

[Foxhound438]

I feel like legendary resistance accomplishes the same goal without making certain character builds feel singled out. If my thing is casting disintegrate a lot, which does no damage on a successful save, my character's been gutted by this, where with legendary resistance there's a certain amount of beating one's head against the wall that can solve it.

[Eldariel]

That would qualitatively not really matter. All it does is make casters rely on their no-save effects like Walls of Force, Forcecages, Animate Objects, etc. more (which were already the safest options). Or just True Polymorph/Shapechange/Magic Jar into something that can hurt them seriously. In short, it pushes casters against the safer, more optimal strategies that were already more optimal. Hurts the less optimal casters, doesn't affect the more optimal ones. I think this does the opposite of what you want, really (much like Legendary Resistance).

[False God]

When I built epic level monsters for 5E, I just gave them more (5) Legendary Resistances and put them on a recharge (5-6). Felt a little more like "working within rules already designed" rather than inventing new ways to say "this guy is extra awesome".