Legendary Resistance: with or without knowledge?

[Cikomyr]

So I was wondering the degree of knowledge a SM should request for its use of Legendary Resistance.

Like, should the DM request to know what's the spell being used before deciding if he's going to use it? Or should he decide to use it without prior knowledge?

"OK, the boss needs to make a constitution saving throw"

Because maybe the player is seeking to make the boss burn its Legendary Resistances on cantrips or level 1 Spells? Would that be an acceptable strategy, or does the DM always know? Does the boss knows? Is the boss knowledgeable and intelligent to use its legendary Resistance only on Save-or-Suck spells?

[Sigreid]

The way I play it is if the monster fails a save they use their legendary resistance. What they're resisting is irrelevant as I don't treat LR as a conscious power.

[Mith]

The way I play it is if the monster fails a save they use their legendary resistance. What they're resisting is irrelevant as I don't treat LR as a conscious power.

An idea I like for legendary resistance is to have a pool of points to add to saving throws.

So instead of 3 auto saves, they get 30 points which they can spend to pass saves.

[PhoenixPhyre]

The way I play it is if the monster fails a save they use their legendary resistance. What they're resisting is irrelevant as I don't treat LR as a conscious power.

This is my strategy as well. They will pass their first 3 saves either by rolling or by using LR.

Because the whole point of LR is to let them act at least once during the combat. Saving it for "strategic" use doesn't help achieve that activity, and requires giving monsters information that players don't necessarily have (I don't call spells until they fail the save unless it's in-universe obvious).

[Bobthewizard]

I have them know the spell and choose whether or not to save. I'm not going to have it waste a legendary resistance on a damaging cantrip. If you just use the first 3 saves, the players are going to learn to run in and all cast save cantrips to try to get it to waste its resistance.

[stoutstien]

Double blind for me and the party. I don't know the spell and they don't know if I used the NPCs LR. If it lands I let it stick and if I pass they can't just count down the LR then let loose.

[Chronos]

They know whatever they have the means to know. If a red dragon is engulfed in a big burst of fire, it's probably not going to even bother to try. It might or might not know if it's Fireball vs. Firestorm vs. an enlarged Burning Hands, but it knows that it's immune to fire (and the joke's on it if it turns out to be Flamestrike, and it still takes half damage anyway). And someone told recently of hitting a dragon with Hypnotic Pattern, which it didn't bother legendarying against for what seemed to it to be good reasons: Apparently that dragon had enough knowledge of the arcane that it knew what a spell was that manifested as a bunch of pretty swirly colors.

On the other hand, unless it's using Arcana as a reaction, or is reading thoughts, or something, a monster would have no way to tell the difference between Dominate Monster and Tasha's Hideous Laughter.

[BloodSnake'sCha]

The monster like the players get to roll arcana if they are spell casters or have arcana prof.

That is how it rolls in my table.

[Frozenstep]

I've talked about this before, but this is one of the reasons I've changed the Legendary Resistance system in my game.

Monster just blindly uses LR of first 3 saves they fail, and party can see when a monster uses a LR? Party is incentivized to simply spam cantrips until they know the monster is out of LR's. Meta-gamey and feels wrong.

Monster blindly uses LR on first 3 saves they fail, but players don't know if they made the save or had to LR? Still incentivized to used the cantrips, but now they're doing it with an element of blindness, especially if they don't know what saving throws a monster is good at. This does nothing but add a guessing game to when the party should start throwing out save or sucks.

Monster only uses LR on dangerous save-or-sucks? Meta-gamey, and incentivizes the party to focus on spells that either chew through LR's or don't target saves at all. While at first that sounds like it's good because it changes how the party fights, it actually makes everything feel the same, because as soon as you figure out an enemy has LR's you fall back on the same few spells across multiple characters, even if those characters normally have wildly different playstyles from each other normally. It leads to some silly situations like monsters not LRing a fireball because they're more scared of a save or suck, but then they die with LR's unused because why would the caster spend 5~ rounds trying to burn resistances when they could just help the martial kill the enemy by round 4?

Monster only uses LR on things it thinks are dangerous? Seems kind of hard to judge, and from a player perspective does it provide them a choice? Do they choose to use a certain kind of spell because they think it's not as obvious to the LRing creature? How do they know? It still feels meta-gamey because now you're using those spells because you know the LR exists and you're trying to get around it.

I find as soon as you see a legendary action, you basically just hit a switch and suddenly everyone uses the part of their kit that's meant to take care of LR creatures. It's not interesting and always feels off.

Of course, not having such a system is even worse because a single nat 1 can mean a boss dies turn 1, which is why I changed my system rather than remove it.

[stoutstien]

I've talked about this before, but this is one of the reasons I've changed the Legendary Resistance system in my game.

Monster just blindly uses LR of first 3 saves they fail, and party can see when a monster uses a LR? Party is incentivized to simply spam cantrips until they know the monster is out of LR's. Meta-gamey and feels wrong.

Monster blindly uses LR on first 3 saves they fail, but players don't know if they made the save or had to LR? Still incentivized to used the cantrips, but now they're doing it with an element of blindness, especially if they don't know what saving throws a monster is good at. This does nothing but add a guessing game to when the party should start throwing out save or sucks.

Monster only uses LR on dangerous save-or-sucks? Meta-gamey, and incentivizes the party to focus on spells that either chew through LR's or don't target saves at all. While at first that sounds like it's good because it changes how the party fights, it actually makes everything feel the same, because as soon as you figure out an enemy has LR's you fall back on the same few spells across multiple characters, even if those characters normally have wildly different playstyles from each other normally. It leads to some silly situations like monsters not LRing a fireball because they're more scared of a save or suck, but then they die with LR's unused because why would the caster spend 5~ rounds trying to burn resistances when they could just help the martial kill the enemy by round 4?

Monster only uses LR on things it thinks are dangerous? Seems kind of hard to judge, and from a player perspective does it provide them a choice? Do they choose to use a certain kind of spell because they think it's not as obvious to the LRing creature? How do they know? It still feels meta-gamey because now you're using those spells because you know the LR exists and you're trying to get around it.

I find as soon as you see a legendary action, you basically just hit a switch and suddenly everyone uses the part of their kit that's meant to take care of LR creatures. It's not interesting and always feels off.

Of course, not having such a system is even worse because a single nat 1 can mean a boss dies turn 1, which is why I changed my system rather than remove it.

What system are u using now? I'm currently working one one my self the ditches LR in favor of giving NPCs more actions a round to increase the number of chances to save vs effects.

[Frozenstep]

What system are u using now? I'm currently working one one my self the ditches LR in favor of giving NPCs more actions a round to increase the number of chances to save vs effects.

It's very similar to that, actually! I'm currently trying a system where a single legendary action can be used to re-attempt a saving throws/checks to end an effect on themselves, and they can also roll saving throws at a disadvantage against effects that normally do not allow additional saves at the end of turns (banishment).

[stoutstien]

It's very similar to that, actually! I'm currently trying a system where a single legendary action can be used to re-attempt a saving throws/checks to end an effect on themselves, and they can also roll saving throws at a disadvantage against effects that normally do not allow additional saves at the end of turns (banishment).

Interesting. I'm trying to remove LR and just add a LA of repeating a save for one point. I want to give this as the only LA for mid tier bosses and such

[Frozenstep]

Interesting. I'm trying to remove LR and just add a LA of repeating a save for one point. I want to give this as the only LA for mid tier bosses and such

I'm personally thinking about a bit more of the opposite direction, in that LA's are more valuable as part of the boss's offense, and so forcing them to burn it means giving up attacks, meaning so that even if a monster fails a save and then then gets rid of it in the same round with a LA, at least the spellcaster has done something valuable in preventing an attack.

[Bobthewizard]

I don't treat LR the same as Counterspell. Counterspell is something that you decide to use in reaction to a spell being cast. It is used consciously.

I don't treat LR as something the NPC uses consciously. It is a protection that I control as the DM that lets me have them succeed on 3 failed saves that I want them to. It's more of a heightened magic resistance for legendary creatures than something they choose to do.

[Demonslayer666]

I play Legendary Resistance as the monster gets to decide when to use it. I have told my players to pause if they expect a reaction like counterspell. I make my decision then. If they blurt what they are doing, and the creature does not know what's coming, I usually do a coin flip if they are using LR.

I also hint to them when they made the creature burn a legendary resistance.

[Cikomyr]

How about "Legendary Resistance: X"

Where X is the amount of spell level the boss can Legendary Resist.

Not sure of the balance really. Maybe make it (Number of LR the monster has x Proficiency Bonus of the Party)

So a monster with 10 Legendary Resist can swat away 10 Cantrips, or 2x Level 5 spells. So if you want to burn through that resistance fast, you still need high spell levels?

Edit: sorry. Cantrips count as 1

[PhoenixPhyre]

How about "Legendary Resistance: X"

Where X is the amount of spell level the boss can Legendary Resist.

Not sure of the balance really. Maybe make it (Number of LR the monster has x Proficiency Bonus of the Party)

So a monster with 10 Legendary Resist can swat away 10 Cantrips, or 2x Level 5 spells. So if you want to burn through that resistance fast, you still need high spell levels?

Edit: sorry. Cantrips count as 1

Doesn't work with all the non-spell way to inflict saving throws. The biggest of which is a monk's Stunning Strike, which might burn out all the LR a monster has in a single turn.

[Trickery]

I don't like Legendary Resistance. Instead, I just make creatures immune to things they should be immune to - ex: paralysis, stun, and so on for a giant boss creature. And I let the players know when that happened, too, in the same way they would know if they were doing damage with their attacks. "You felt your spell take hold, but it had no effect."

[zinycor]

The way I play it is if the monster fails a save they use their legendary resistance. What they're resisting is irrelevant as I don't treat LR as a conscious power.

I do it like that as well

[jas61292]

A lot of people here seem to not like thre creature getting to choose when to use their resistance, but to me, that is its redeeming quality as a feature. Making it automatic turns it into a metagame ability for the players to cheese. Letting the creature pick makes it an actually interesting and useful feature to protect a creature from a quick, anticlimactic death.

Narratively, the way I think of it is not as something subconscious, but rather them actively shaking off something that did, in fact, effect them. Kinda like the a second wind, but stronger. It's not them automatically succeeding against that dominate spell unconsciously, but them starting to feel it's mind being controlled, and having the legendary ability to be able to shake that off before it fully loses control. This also nicely ties into the mechanical fact that it happens after a failed save, not by making a save without rolling it.

With that said, as with much of how monsters are played, I think it's important to think about how a monster would react to effects, and what it would prioritize using the ability against. As a DM, before I use a creature with legendary resistance, I always think about this and make a plan. For one creature that might mean using the resistance only against things that would effect its mind or bodily control. For others it might mean it will use them exclusively for damage mitigation. One might use them on the first three effects it can, while another might save them and not use any unless it is already weak. Whatever it is though, it's predetermined and consistent fit that creature. It will never be me arbitrarily deciding that I don't want a specific player ability to work, on the fly. The only othe rule I have is that, unless otherwise specified in my planning, a creature will always use a legendary resistance if the effect it failed its save against would otherwise kill it.

Personally, I think that without the freedom to use the ability as you want, it becomes far less useful, and far more gamist. And I don't like that.

[darknite]

It's a game and I'm the DM. The players already hold most of the advantages. I have no problem knowing what a legendary creature is saving against when they use an LR. They're legendary creatures, right? It's not like Larry the Goblin is spellcrafting the incoming Disintigrate and choosing to pass, but Maxithrosus, the Whispering Death, and mighty lich, gets that nod of respect to not be a chump and maybe live to fight another round. I find my players prefer it that way.

[Waazraath]

A lot of people here seem to not like thre creature getting to choose when to use their resistance, but to me, that is its redeeming quality as a feature. Making it automatic turns it into a metagame ability for the players to cheese. Letting the creature pick makes it an actually interesting and useful feature to protect a creature from a quick, anticlimactic death.

Narratively, the way I think of it is not as something subconscious, but rather them actively shaking off something that did, in fact, effect them. Kinda like the a second wind, but stronger. It's not them automatically succeeding against that dominate spell unconsciously, but them starting to feel it's mind being controlled, and having the legendary ability to be able to shake that off before it fully loses control. This also nicely ties into the mechanical fact that it happens after a failed save, not by making a save without rolling it.

With that said, as with much of how monsters are played, I think it's important to think about how a monster would react to effects, and what it would prioritize using the ability against. As a DM, before I use a creature with legendary resistance, I always think about this and make a plan. For one creature that might mean using the resistance only against things that would effect its mind or bodily control. For others it might mean it will use them exclusively for damage mitigation. One might use them on the first three effects it can, while another might save them and not use any unless it is already weak. Whatever it is though, it's predetermined and consistent fit that creature. It will never be me arbitrarily deciding that I don't want a specific player ability to work, on the fly. The only othe rule I have is that, unless otherwise specified in my planning, a creature will always use a legendary resistance if the effect it failed its save against would otherwise kill it.

Personally, I think that without the freedom to use the ability as you want, it becomes far less useful, and far more gamist. And I don't like that.

Feel like this as well. So I know what the monster is saving against, and player's only know that the monster succeeded, wether it made a save or used a LR.

On a side note: I'm away from books, but the DM always knows what spell is cast, doesn't (s)he? The player states his or her action, that includes "cast X spell, with Y target".

[Trickery]

On a side note: I'm away from books, but the DM always knows what spell is cast, doesn't (s)he? The player states his or her action, that includes "cast X spell, with Y target".

That's been my experience. However, I've also usually played with DMs who told the player what was being cast. It gets pretty clunky when they don't.

DM: make a dex save.
Player: against what?
DM: don't worry about it, just make the save.
Player: I kind of need to know because of feature X.
DM: oh, what does that do?
Player: ...

You get the idea. It's the same problem as Dungeon Delver has. In order to roll to detect hidden doors with advantage, I need to know that's what I'm doing. So I guess I just make that roll before entering the room? Otherwise the DM has to tell me when he asks for a perception check that it's to detect the presence of a secret door, at which point everyone starts tapping on the walls, pulling up rugs, and moving bookcases. Basically, the player needs to know something they aren't supposed to know to use their features.

[Aimeryan]

A lot of people here seem to not like thre creature getting to choose when to use their resistance, but to me, that is its redeeming quality as a feature. Making it automatic turns it into a metagame ability for the players to cheese. Letting the creature pick makes it an actually interesting and useful feature to protect a creature from a quick, anticlimactic death.

Narratively, the way I think of it is not as something subconscious, but rather them actively shaking off something that did, in fact, effect them. Kinda like the a second wind, but stronger. It's not them automatically succeeding against that dominate spell unconsciously, but them starting to feel it's mind being controlled, and having the legendary ability to be able to shake that off before it fully loses control. This also nicely ties into the mechanical fact that it happens after a failed save, not by making a save without rolling it.

With that said, as with much of how monsters are played, I think it's important to think about how a monster would react to effects, and what it would prioritize using the ability against. As a DM, before I use a creature with legendary resistance, I always think about this and make a plan. For one creature that might mean using the resistance only against things that would effect its mind or bodily control. For others it might mean it will use them exclusively for damage mitigation. One might use them on the first three effects it can, while another might save them and not use any unless it is already weak. Whatever it is though, it's predetermined and consistent fit that creature. It will never be me arbitrarily deciding that I don't want a specific player ability to work, on the fly. The only othe rule I have is that, unless otherwise specified in my planning, a creature will always use a legendary resistance if the effect it failed its save against would otherwise kill it.

Personally, I think that without the freedom to use the ability as you want, it becomes far less useful, and far more gamist. And I don't like that.

It comes down to why you think they have LR in the first place. If you want to just use up caster's resources then being able to save it for when they pull out the big guns makes sense. However, I see the design as being in place so that the fight lasts longer in a useful state, which HP alone would not do. In that case, what they use the LR on doesn't matter, just that the fight lasts longer.

If I was to modify it, I would probably look at some form of status resistance and diminishing returns, along with more HP. The fight should last longer without being 'use up the LR, then win'.

[Lunali]

If you always tell players what they're saving against, the monster should get the same benefit. If you don't tell players until after the save is made or failed, the monster should have the same limit.

[stoutstien]

I don't like Legendary Resistance. Instead, I just make creatures immune to things they should be immune to - ex: paralysis, stun, and so on for a giant boss creature. And I let the players know when that happened, too, in the same way they would know if they were doing damage with their attacks. "You felt your spell take hold, but it had no effect."
Bye bye to the monk's best class feature.
we should reward players who try CC or debuffs vs just flat damage. I feel this does the opposite.

[Trickery]

Bye bye to the monk's best class feature.
we should reward players who try CC or debuffs vs just flat damage. I feel this does the opposite.

Most creatures aren't giant boss monsters. How many of those do you fight per campaign, and how many of them do you suppose have backup?

[Aimeryan]

Bye bye to the monk's best class feature.
we should reward players who try CC or debuffs vs just flat damage. I feel this does the opposite.

This reminds me of the Final Fantasy games where everything that actually mattered was immune to pretty much all of the status conditions. The result was you just always focused on damage and the status conditions might as well have not existed as tools for the player.

My preferred way of handling status conditions and bosses is for them to have a resistance that is chipped away at either by time or some other measure (HP, maybe) - then the player can try the 'big guns' once they think the cost is worth it. Legendary Resistance does this, but its very clunky - it often feels like it is just 'waste your time and resources for a bit, then you get to play'. It really should be 'use your time and resources for a bit doing productive stuff, then you can try to pull off big things'. Of course you can, and should, be looking at doing things in the fight that is not reliant on saves - Walls, teleports, Absorb Elements, Heal, etc - however, this doesn't help in anyway at all in getting past LR.

To summarise, this is how it should look in my opinion, in order of importance:
1. Stop boss instantly sucking due to Save-or-Suck.
2. Allow boss to eventually become vulnerable to sucking due to Save-or-Suck.
3. Allow productive activity to bring this about.

LR manages 1 and 2, fails on 3.

[Nagog]

Dragons are inherently magical creatures, particularly if they're powerful enough to have LR. Odds are it will recognize a spell for what it is, and know the effects of it.

However things change id Subtle Spell is involved, or if it's something without a visible effect cast from stealth (Arcane Tricksters even have an ability to impose disadvantage on saves under these conditions, which may be enough to tease out those LR with cantrips if executed properly. If the Dragon has True Seeing, you'll definitely need Subtle Spell.)

[stoutstien]

Most creatures aren't giant boss monsters. How many of those do you fight per campaign, and how many of them do you suppose have backup?
I like them a lot and so do players 😀. Id say every other session has one of one sort or an other

[Lunali]

Dragons are inherently magical creatures, particularly if they're powerful enough to have LR. Odds are it will recognize a spell for what it is, and know the effects of it.

However things change id Subtle Spell is involved, or if it's something without a visible effect cast from stealth (Arcane Tricksters even have an ability to impose disadvantage on saves under these conditions, which may be enough to tease out those LR with cantrips if executed properly. If the Dragon has True Seeing, you'll definitely need Subtle Spell.)

True Seeing in no way allows you to see creatures that are hidden unless the only thing allowing them to hide is invisibility or something similar.

[Trickery]

I like them a lot and so do players 😀. Id say every other session has one of one sort or an other

Then yours probably shouldn't have condition immunities. I only use those for special bosses.

[stoutstien]

Then yours probably shouldn't have condition immunities. I only use those for special bosses.

I see resistance and immunity as flavor over function abd should be pretty predicable. Red dragons are immune to fear, undead hate radiant, don't stab skeletons.
I'd rather build NPCs to be able to be effected by the CC and debuffs but in way that doesn't completely remove it's ablity to act.
Slapping immunity to all the character's most effective options a d calling it good isn't fun for them or me.

[Trickery]

I see resistance and immunity as flavor over function abd should be pretty predicable. Red dragons are immune to fear, undead hate radiant, don't stab skeletons.
I'd rather build NPCs to be able to be effected by the CC and debuffs but in way that doesn't completely remove it's ablity to act.
Slapping immunity to all the character's most effective options a d calling it good isn't fun for them or me.

DMing for a party full of high damage characters, one of the most memorable encounters I ever threw at them was a boss that couldn't be damaged. They had to get creative. There's nothing wrong with throwing a challenge at the players that their usual strategies won't work on.

[stoutstien]

DMing for a party full of high damage characters, one of the most memorable encounters I ever threw at them was a boss that couldn't be damaged. They had to get creative. There's nothing wrong with throwing a challenge at the players that their usual strategies won't work on.

I'm Sure it was. Once. The occasional one is fine. What I wouldn't recommending is just slapping immunies on every boss. Especially stun which is supposed to be a rare immunity due to how rare the effect is. Sure the monk will stun stuff. It's the classes Main way of helping the the party vs hard foes. Better off just turning stun to emulate the effects of the slow spell if you are worried it's too much.

[Lord Vukodlak]

At my groups table it’s done with knowledge, we don’t run spell identification rules so you know what’s being cast, unless the caster can hide the fact their casting or somehow obscure the details.

[Tawmis]

So I was wondering the degree of knowledge a SM should request for its use of Legendary Resistance.
Like, should the DM request to know what's the spell being used before deciding if he's going to use it? Or should he decide to use it without prior knowledge?
"OK, the boss needs to make a constitution saving throw"
Because maybe the player is seeking to make the boss burn its Legendary Resistances on cantrips or level 1 Spells? Would that be an acceptable strategy, or does the DM always know? Does the boss knows? Is the boss knowledgeable and intelligent to use its legendary Resistance only on Save-or-Suck spells?

I always ask the players to describe the spell being used - this lets everyone at the table know what's happening.
Also gives a great description (if the player is new to D&D) as to what that spell is doing.
When it targets a creature that has Legendary Resistance, I have the monster (me) choose when to make those Legendary Resistances.
For the same reason everyone mentioned - if players know An Adult Red Dragon (https://www.dndbeyond.com/monsters/adult-red-dragon) has 3 legendary resistances to use; I don't want them throwing low level cantrips at it to burn those. The dragon will know when its about to be hit by something potentially big - and saves against it. To be fair, I announce the Legendary Resistance before the damage is rolled. For example if a Wizard comes in and hits the dragon with Uther's Ultimate Chain Lightning Strike which does 10d100 damage - I will use (as an example) a Legendary Resistance before the Wizard begins rolling damage (where normally it's full or half based on saves - Legendary nulls it all). That way players know I am not like, "Oh, you rolled 82 points of damage? So. Yeah. Krymson the Red decides to use Legendary Resistance." That would not be cool.

[Lord Vukodlak]

(where normally it's full or half based on saves - Legendary nulls it all). That way players know I am not like, "Oh, you rolled 82 points of damage? So. Yeah. Krymson the Red decides to use Legendary Resistance." That would not be cool.

Excuse me what!? That's not what legendary resistance does if you sail a saving throw you can choose to succeed instead.

[darknite]

DMing for a party full of high damage characters, one of the most memorable encounters I ever threw at them was a boss that couldn't be damaged. They had to get creative. There's nothing wrong with throwing a challenge at the players that their usual strategies won't work on.

I agree with this a lot. Omnicient PCs make for boring battles. Disrupting their usual strategies makes them observe causes and effects, dig deep and get creative. Bosses that "couldn't be damaged" is a bit rough, but as long as there was an accessible way to win, I'm sure it was a blast.

[Trickery]

I agree with this a lot. Omnicient PCs make for boring battles. Disrupting their usual strategies makes them observe causes and effects, dig deep and get creative. Bosses that "couldn't be damaged" is a bit rough, but as long as there was an accessible way to win, I'm sure it was a blast.

There was - I'm not evil =). It didn't move too quickly and was susceptible to other effects. The players just had to stop it, not kill it. It came back later in the campaign. Fun was had.

[hymer]

I'd definitely say the DM knows what is being cast. Keeping things from the arbiter of the game just makes it harder to get things right, and leads to all manner of trouble. Suppose a PC uses a poison effect on a poison immune monster. Do they then get to take it back when they realize the monster is immune? Do they get to see you roll to save and announce the result, so they now know the monster's Con save modifier? What if there is some effect at work that the PCs are not aware of, like a Nondetection or Contingency spell?

No, let's keep it simple: You say what spell your PC is casting, and the DM will handle Legendary Resistance according to their style.

[Aimeryan]

I'd definitely say the DM knows what is being cast. Keeping things from the arbiter of the game just makes it harder to get things right, and leads to all manner of trouble. Suppose a PC uses a poison effect on a poison immune monster. Do they then get to take it back when they realize the monster is immune? Do they get to see you roll to save and announce the result, so they now know the monster's Con save modifier? What if there is some effect at work that the PCs are not aware of, like a Nondetection or Contingency spell?

No, let's keep it simple: You say what spell your PC is casting, and the DM will handle Legendary Resistance according to their style.

You can do the whole Saving Throw part before revealing the spell - it alters nothing.

[Tawmis]

Excuse me what!? That's not what legendary resistance does if you sail a saving throw you can choose to succeed instead.

Right - I wasn't very clear apparently.
So on a spell that does either full damage on a failed Savings Throw and 1/2 Damage on a Successful Throw - if a Legendary Action is used, it nullifies "failing" (so that you would not take full damage, but rather 1/2 damage instead).