Firstly, you're right. I shouldn't have typed out the disadvantage on attack rolls. I was letting multiple pieces of information bleed together there.
But, crowd control isn't always used against things that you can or want to kill. This spell is really good at messing with a crowd's ability to handle an area that you want to lock down while still being able to use the area for yourself and your allies.
Why is Wisdom save terrible? Is a Wisdom saving throw terrible for all spells that require them?

While it's true that roughly ¼ of monsters on DNDBeyond are immune to the frightened condition, I don't think that invalidates the usefulness of a spell in the right situation. All spells have their value over others, whether that's because of the spell slot expense, effectiveness, or because of situational value. I'm always fond of the ✌️bad spells✌️, 'cause I like to see how they can situationally be the best option.

The note on DM tools is also an excellent one too. Not all spells are best used by players. Instant summons, magic mouth, continual flame, and most illusion spells being prime examples.

I hadn't done my homework and was assuming most illusion spells were intelligence saves, but they're actually largely Wisdom. It would still be better if it was intelligence mathematically, but it's at least consistent with the "fear" effects that exist already, I was wrong. Everything else still holds, though—like, you say it's good for messing with a crowd but it doesn't even discern friend from foe for a 9th level slot. I can respect wanting to find uses for the so called bad spells, but having a single use case doesn't make them not bad. Like, this spell essentially gives the targets advantage on the (low) damage effect. It's just so mind-numbingly bad. If you cast it on me I would see screenshots of this description being touted as a good 9th level spell.

No, but, seriously, play however you want, if you have fun with this spell that is excellent. I think it's lacking compared to other 9th level options like turning into an adult dragon, getting advantage on literally everything, being immune to all damage, stopping time, and Psychic Scream, which is literally this but with all of the bad aspects cut away:

Wider area (90 feet centered on you), discerns friend from foe, more than twice as much average damage, only 1 int save initially, which deals half-damage on a success, stunned condition which is insanely strong, and no concentration. Oh, and it's also more than just a Wizard exclusive, not that Sorcerers were sitting around fuming over not being able to take Weird. 

While it's true that the spell doens't discern friend from foe, it des allow friends to move into the affected area without and threat to themselves after it's been cast, while the spell can still persist, and that's pretty phenomenal in addition the the effects and damage. Like most AOEs, it's a bad spell if used against a single target, and more powerful AOEs just get worse at that because they usually get bigger.

I'd argue that having a single or few cases where a spell is great, doesn't make a spell bad. It just means it's the wrong tool for what else you'd want it for. A wizard is like a mechanic or another specialist builder/artisan. A single tool in a tool-kit might be almost never used, but it's very rewarding to have it when it is the best tool for a job at hand.

If by "essentially giving advantage" you mean that it takes two saving throws to deal damage, I'd say that because the spell is primarily a Fear-effect spell, and the possibility of dealing damage with one makes it a 9th level option. I don't disagree that it'd be nicer to have the spell be lower level, or that it would be too strong if it were of a lower level, but comparing it to all other spells than inflict the frightened condition, especially the ones a Wizard can cast, this is still the most powerful one. Even without dealing damage, it's still be the most powerful one, but it gets possible damage added to the primary function of being a condition-application spell for battlefield control.
     Cast before your allies charge in, and then let your allies lay waste to your enemies while they're unable to move or effectively fight back.

A lot of 9th level spells are what I'd call war spells, because they have the power to ruin entire armies. This, honestly would do a good job of giving your side of the army a huge starting advantage that doesn't just come out of damage. Worth noting that while Psychic Scream can only hit up to 10 creatures. It's definitely a more versatile spell than Weird.

Quote from Jety_Lefr >>

While it's true that the spell doens't discern friend from foe, it des allow friends to move into the affected area without and threat to themselves after it's been cast, while the spell can still persist, and that's pretty phenomenal in addition the the effects and damage. Like most AOEs, it's a bad spell if used against a single target, and more powerful AOEs just get worse at that because they usually get bigger.

I'd argue that having a single or few cases where a spell is great, doesn't make a spell bad. It just means it's the wrong tool for what else you'd want it for. A wizard is like a mechanic or another specialist builder/artisan. A single tool in a tool-kit might be almost never used, but it's very rewarding to have it when it is the best tool for a job at hand.

If by "essentially giving advantage" you mean that it takes two saving throws to deal damage, I'd say that because the spell is primarily a Fear-effect spell, and the possibility of dealing damage with one makes it a 9th level option. I don't disagree that it'd be nicer to have the spell be lower level, or that it would be too strong if it were of a lower level, but comparing it to all other spells than inflict the frightened condition, especially the ones a Wizard can cast, this is still the most powerful one. Even without dealing damage, it's still be the most powerful one, but it gets possible damage added to the primary function of being a condition-application spell for battlefield control.
     Cast before your allies charge in, and then let your allies lay waste to your enemies while they're unable to move or effectively fight back.

A lot of 9th level spells are what I'd call war spells, because they have the power to ruin entire armies. This, honestly would do a good job of giving your side of the army a huge starting advantage that doesn't just come out of damage. Worth noting that while Psychic Scream can only hit up to 10 creatures. It's definitely a more versatile spell than Weird.

 Armies generally occupy more than a 30 foot space.

Quote from Jety_Lefr >>

I don't know why everyone hates on this spell. Fear is a powerful effect to bestow, and the fact that the source of the fear is implacable, means that targets can't move while afraid. The targets that fail the save can't move, have disadvantage on attack rolls, while also taking damage while the spell lasts.
And it can effect a lot of creatures from a safe casting distance.

It's a very good spell for well-rounded combat control. The fact that it's so high level also means that the caster automatically has at least a +6 to the DC just from proficiency, so the DC is likely to be 18 or 19 by the time a caster can use this spell. Even if it ends on some targets, it can persist on many others. It's a useful well-rounded control spell for combat, with some damage added. Yes, instantaneous damage that is higher deals more damage, but that doesn't do anything to lock down and debuff targets.

This spell is good for party-cooperation.

I think you are confusing the word implacable (meaning unable to be placated) with unplaceable (meaning unable to be placed).

Though, even if the word used was unplaceable there is nothing RAW that supports your assertion that the target would be unable to move. The frightened condition says they cannot willingly move closer to the source of their fear - if they did know know the location of the source of their fear that means they are free to move whereever they'd like. It would also mean they don't have disadvantage at all because that requires them to be in line of sight of the source which again wouldn't apply if they couldn't see that source so under that reading the spell would literally do nothing. 

Quote from Abelhawk >>

This should really be an Intelligence saving throw instead of Wisdom, since it's dealing with illusions. I would also rule that those who fail the first save take the damage and those who succeed take half damage.

Quote from LichKingNerzhul >>

I hadn't done my homework and was assuming most illusion spells were intelligence saves, but they're actually largely Wisdom. It would still be better if it was intelligence mathematically, but it's at least consistent with the "fear" effects that exist already, I was wrong. Everything else still holds, though—like, you say it's good for messing with a crowd but it doesn't even discern friend from foe for a 9th level slot. I can respect wanting to find uses for the so called bad spells, but having a single use case doesn't make them not bad. Like, this spell essentially gives the targets advantage on the (low) damage effect. It's just so mind-numbingly bad. If you cast it on me I would see screenshots of this description being touted as a good 9th level spell.

No, but, seriously, play however you want, if you have fun with this spell that is excellent. I think it's lacking compared to other 9th level options like turning into an adult dragon, getting advantage on literally everything, being immune to all damage, stopping time, and Psychic Scream, which is literally this but with all of the bad aspects cut away:

Wider area (90 feet centered on you), discerns friend from foe, more than twice as much average damage, only 1 int save initially, which deals half-damage on a success, stunned condition which is insanely strong, and no concentration. Oh, and it's also more than just a Wizard exclusive, not that Sorcerers were sitting around fuming over not being able to take Weird. 

No, Wisdom is the correct save against illusions. Wisdome is used for both, perception and insight after all. So Wisdom is used to realize you're dealing with an illusion. Intelligence would be to figure out how that illusion works and what it does. 

I would argue that since the spell is what frightens them that the caster counts as source of their fear. I agree the spell sucks, but I would rule that they would be unable to move towards the caster. 

This spell should warp reality so that it's more like the plane Limbo. Now, THAT would be a spell worthy of the name "Weird". This is just a glorified Fear spell. It should melt stone that freezes into glass that snows down into adamantine horseshoes and stuff. Think about how useful that could be in combat (and for other reasons)!!!!

Agree this spell is not that great, but on the list of bad spells either too powerful, or not powerful enough it is like ranked 38th.  Indeed, Weird used to potentially insta-kill.

However, my House rule is offensive spells that require saving throws, require no concentration.  It helps spells like these a bit.  Also, the should be upon failing the save immediately.  In addition Pound for pound, psychic damage is good.  Feels nonetheless, more like 5th or 6th level.

This is the worst 9th level spell by far.

Quote from Whatdisdis >>

Any advice on how to buff this spell?

 Replace it (and Phantasmal Killer) with their 3.5e counterparts or something like it. Wisdom save to determine who's able to see through the illusion, then a Con save to avoid dying instantly if you fail the Wisdom save. Have it do damage on a successful Con save (though not on a successful Wisdom save).

Is worse in nearly every way compared to the 4th level spell sickening radiance. This spell inflicts fear, which is much less potent than exhaustion. Additionally, sickening radiance, as the name suggests, inflicts radiant damage instead of psychic. Not only is radiant damage less resisted than psychic damage (especially so considering most creatures with radiant resistance probably won't want to fight you anyways), but many creatures are also weak to radiant damage. sickening radiance even comes with the bonus of making creature lose the benefits of invisibility. The only possible upside of this spell compared to sickening radiance is that it targets WIS instead of CON, as many creatures have good con saves. Not that WIS is a super uncommon save either, and just targeting WIS is obviously not worth a difference of 5 levels.

AoE phantasmal killer...

Like any other illusion spell, a lot depends on your creativity and a discussion with your DM.  Respecting RAW but wanting this to be a "good" 9th level spell is a pretty valid place to start that discussion.  I find it valuable to think of illusions as leaning more heavily on the "role playing" part of RPG than other spells.  So as a DM, I've leaned into the fact that the targets see the creatures (plural, not singular) in their minds and are their "worst nightmares" and "implacable." 

Consider playing it like this; not a focus on MECHANICS, but on BEHAVIOR.  If a creature fails that first saving throw, the "cool" part of the spell is the creatures behavior the next round changes.  It's surrounded by multiple versions of its worst nightmare!  It is absolutely convinced of it's inadequacy against these foes, and the lethal threat they pose.  "Targeting priority" shifts.  Previous instructions or planning are thrown out the window in a panic.  I do rule that the affected creatures can move (nothing in the spell description says that they are immobilized or paralyzed by their fear) but I think of it as movie/nightmare logic- they can run to try and escape but keep seeing the nightmares around them wherever they look, so the frightened condition can't drop due to movement.

The affected creature ignores the true enemies on the field (players) and unloads the biggest attack/spell/ability it can at illusory foes.  Nothing held back- it would even flee the battle entirely if it has the means (etherealness, plane shift, teleport, etc) to do so.  The creature now wasted a turn, and will possibly waste more.  In that context, an extra 4d10 damage is the gravy, not the main course.

Other factors to consider roleplaying as the DM (or advocating for, if you are the player)- does the affected creature drop AoE effects that hurt allies?  I've even played affected creatures dropping AoE attacks on top of themselves in a desperate move to kill the source of fear, or "sacrifice" themselves to stop the illusory enemies from threatening its fellows.  Remember- implacable worst nightmare.  And I take some DM liberty to rule that it sometimes sees those multiple copies in adjacent spaces, because who's ever had a nightmare where the threat is 300 ft away?

Also, given the pants-wetting terror the targets are under, members of the PC party should be allowed to easily discern who's affected and clear the line of fire, no role required.

Have fun!    

This is inarguably the single worst, most useless, ineffectual spell above 6th level. The fact that this costs a 9th level slot is laughable. Here's the breakdown:

1. "Each creature in a 30-foot radius..." All of them. NOT "each creature of your choice", they said "each creature". That means this spell hits your allies and, potentially, yourself. For a 9th level spell, we're off to a real bad start.
2. "...make a Wisdom saving throw. On a failed save, a creature becomes frightened for the duration." It is pitifully easy to gain advantage on Wisdom saving throws, and even easier to give yourself immunity to the frightened condition. For a 1st level slot, Heroism will give one person immunity to the frightened condition, making them immune to this 9th level spell. For a 3rd level slot, Beacon of Hope will give every ally within 30 feet of the caster advantage on this saving throw, effectively doubling their odds at succeeding and rendering this 9th level spell wasted. At 10th level Paladin renders allies within 10 feet immune to being frightened just by being alive and awake.
3. "At the end of each of the frightened creature's turns..." Meaning, if the creature has lost the frightened condition prior to the end of their turn, they are no longer a frightened creature, and they need not make this save-- the spell has effectively ended on them. Every monk just became immune to the damage of this 9th level spell.
4. "...must succeed on a Wisdom saving throw or..." A second Wisdom saving throw. A creature that isn't immune to the frightened condition still has to fail TWO Wisdom saving throws before this spell does any damage at all.
5. "...4d10 psychic damage." IF a creature in the area isn't immune to being frightened, and IF they happened to fail two saving throws that they (statistically) likely had advantage on, then and only then do they take an impotent 4d10 damage. The SAME amount of damage, mind you, as Phantasmal Killer-- a 4th level spell, which only costs a 4th level spell slot.

This spell is best used by a 17th level caster against a swath of nameless NPC fodder, just to showcase the fact that they (1) have spells of this level, and (2) can afford to expend their power wastefully because they have oh-so-much of it.

Attempts to use this spell against any number of competent foes equates to asking for a slot to be wasted. You might as well upcast Shield.

This is how I'd rewrite the spell to be somewhat functional without changing too much (flavor text removed):

Each creature of your choice in a 30-foot-radius sphere centered on a point of your choice within range must make an Intelligence saving throw. On a failed save, a creature becomes frightened for the duration. At the start of a frightened creature's turn, it takes 6d10 psychic damage. A creature can repeat the saving throw at the end of each of its turns. On a successful save, the spell ends for that creature.

Creatures immune to the Frightened Condition have advantage on their saving throw against this spell. On a failure, that creature is Frightened, but it does not take damage from the spell. 

As written, though, the spell is terrible. Why? Well, first, what is good about it?

Well, I think it's a decent spell to use on your players when the BBEG wants to toy with them but not kill them. Since it manifests the character's worst nightmares, there's some interesting RP potential for a creative DM. While everyone is frightened, the bad guy can get away. This won't work on higher level characters who can buff with things like Hero's Feast, but the DM could still pull this off on an unprepared party.

As for a player, it's a good crowd control. While the spell does target your allies, by level 17, it's pretty easy to make your friends immune to fear & therefore this spell. With the massive range and radius, you can immobilize an entire hoard of minions who were supposed to be a big issue in situations where Meteor Swarm isn't a good option. Disadvantage plus 0 movement speed (I think the intent is the source of the fear is everywhere) means most enemies aren't hitting level 17+ characters.
Psychic Scream would still be a better crowd control spell in most situations, but Psychic Scream didn't exist when this spell was written.

But, why is bad?

• It's a 9th level spell. You get one 9th level spell slot a day.

• Friendly Fire: Even if you can prep your allies with fear immunity, that shouldn't be a requirement to make a 9th level spell function properly. This spell is about imposing fear which will hurt your party much more than a bit of friendly fire damage.

• Saving Throw: Many creature's have decent to good Wisdom saving throws, so the likelihood of success is higher than if this were Intelligence.

• Damage: The damage is terrible. An enemy has to first fail 2 saving throws, and then they take 4d10 damage, which is an average of 22 points. This is assuming, also, that you haven't dropped concentration in the meantime.

• Frightened Condition: Imposing fear is the primary draw of this spell. Aside from poisoned and maybe charmed, Frightened is the most commonly negated condition in the game.

• It's a 9th level spell. You could give an ally advantage on everything, turn into a dragon, explode up to 10 enemy's heads, travel anywhere in the multiverse you can name, or cast Wish. Why would you do a mass fear instead?

Just as a comparable, Sunburst is an 8th level spell and it:

• Deals more damage (an average of 42 versus Weird's 22). You'd need an enemy to fail 4 saving throws before Weird outdamages Sunburst in a notable way.
• Has a slightly better range.
• Imposes a more detrimental condition in most circumstances (blinded versus frightened), which is admittedly a bit more resisted when you consider the number of creatures with blindsight/tremor sense.
• Doesn't requires concentration.
• Is an 8th level spell slot.

And Sunburst really isn't that great either.

It seems to me that even on a successful save the illusory creatures are created in the targets' minds, and that these creatures are still manifestations of the targets' worst nightmares. The targets just don't get the frightened condition or have to save each turn to avoid damage. So, if every target makes the initial save, for the spell duration there's a decent chance they'll be moving toward and attacking illusions, giving this spell a rough approximation of the Confusion spell.

Am I reading this right?