This is the best best spell name I have ever found, and the effects are even better.

How is this a 9th level spell? Are there historical reasons perhaps from earlier editions? 

No, this spell is just a bad 9th-level spell that has more thematic usefulness than practical uses. Why force saving throws when you can just cast meteor swarm?

 Weird used to essentially be, Mass Phantasmal Killer, which as the name implies was a save or die spell, and if you did save you still took some damage and were stunned for a round

This is a horrible 9th level spell. Each creature it affects gets 2 saving throws before any damage is done at all. One saving throw when the spell is cast, and one saving throw at the end of the the creature's turn. Also there is no half damage on a success. Even with low wisdom, there's a good chance that a creature will receive no damage from this spell. The mass fear effect could be useful depending on how the DM plays it out, but even then this isn't worth a 9th level spell slot. Also since it effects all creatures, and not chosen creatures, you probably won't be able to target all enemies most of the time because you'll have to be careful to not target your allies.

If an illusion wizard casts this, can they use Illusory Reality to make the targets "deepest fears" manifest for one minute? So even if they make the second save, they'd still have their "deepest fears" made reality? (Assuming it's some kind of object)

Illusory Reality

By 14th level, you have learned the secret of weaving shadow magic into your illusions to give them a semi-reality. When you cast an illusion spell of 1st level or higher, you can choose one inanimate, nonmagical object that is part of the illusion and make that object real. You can do this on your turn as a bonus action while the spell is ongoing. The object remains real for 1 minute. For example, you can create an illusion of a bridge over a chasm and then make it real long enough for your allies to cross.

The object can’t deal damage or otherwise directly harm anyone.

Quote from TheNullStranger >>

If an illusion wizard casts this, can they use Illusory Reality to make the targets "deepest fears" manifest for one minute? So even if they make the second save, they'd still have their "deepest fears" made reality? (Assuming it's some kind of object)

Illusory Reality

By 14th level, you have learned the secret of weaving shadow magic into your illusions to give them a semi-reality. When you cast an illusion spell of 1st level or higher, you can choose one inanimate, nonmagical object that is part of the illusion and make that object real. You can do this on your turn as a bonus action while the spell is ongoing. The object remains real for 1 minute. For example, you can create an illusion of a bridge over a chasm and then make it real long enough for your allies to cross.

The object can’t deal damage or otherwise directly harm anyone.

 It seems like you can do it for one of them, but it seems rather pointless considering it can do no damage, and they would no longer be subject to the frightened condition by RAW

WOTC has really shafted all Illusion spells and the Illusionist wizard so bad. Its on par with the Beastmaster Ranger. Because its so up to interpretation on how illusions work, ive been shafted by every single DM that I've played with when using my Illusionist. 

Look at Psychic Scream, another 9th lvl spell, does more damage and even on a successful save, they still take half damage. This spell? a measly 4d10 and a successful save nothing happens. 

fireball is better than this. Evocation wisards are extremly OP in 5e. 8d8 is better than 4d10. In conclusion, USE METEOR SWARM! IT IS SO MUCH BETTER!

Quote from Darth_Reaver >>

WOTC has really shafted all Illusion spells and the Illusionist wizard so bad. Its on par with the Beastmaster Ranger. Because its so up to interpretation on how illusions work, ive been shafted by every single DM that I've played with when using my Illusionist. 

Look at Psychic Scream, another 9th lvl spell, does more damage and even on a successful save, they still take half damage. This spell? a measly 4d10 and a successful save nothing happens. 

Many DM's are notoriously weirded out by Illusion magic. There is a strain of them that say 'Everybody knows about illusions, so they don't do anything.' If it doesnt' have a hard mechanic, they can't handle it, and think of trickery or using one's brain instead of muscles is 'cheating'. WoTC isn't helping by nerfing illusions. I'm pretty sure that the design team leads are kind of like this. They seem to be allergic to creativity or clever applications of rules, so I'm not surprised. If it's not a sloped-brow bashup, they can't hang.

Hands down the most useless 9th level spell in existence. Fear at level 3 does almost as much. Phantasmal Killer (level 4) essentially is this spell.

Don't you WISH you had picked a different spell?  'Cause, now, WISH - that's not broken. Here's my WISH to WoTC: completely rewrite WEIRD. Read "The Weird of Avoosl Wuthoqquan" [Clark Ashton Smith, 1932] to get in the mindset. It should be a nearly inescapable and deadly prophecy of doom.

Quote from TheNullStranger >>

If an illusion wizard casts this, can they use Illusory Reality to make the targets "deepest fears" manifest for one minute? So even if they make the second save, they'd still have their "deepest fears" made reality? (Assuming it's some kind of object)

Illusory Reality

By 14th level, you have learned the secret of weaving shadow magic into your illusions to give them a semi-reality. When you cast an illusion spell of 1st level or higher, you can choose one inanimate, nonmagical object that is part of the illusion and make that object real. You can do this on your turn as a bonus action while the spell is ongoing. The object remains real for 1 minute. For example, you can create an illusion of a bridge over a chasm and then make it real long enough for your allies to cross.

The object can’t deal damage or otherwise directly harm anyone.

 If the object was harmless and nonmagical...

Quote from Darth_Reaver >>

WOTC has really shafted all Illusion spells and the Illusionist wizard so bad. Its on par with the Beastmaster Ranger. Because its so up to interpretation on how illusions work, ive been shafted by every single DM that I've played with when using my Illusionist. 

Look at Psychic Scream, another 9th lvl spell, does more damage and even on a successful save, they still take half damage. This spell? a measly 4d10 and a successful save nothing happens. 

 Are you forgetting about the frightened effect? Frightened is an extremely powerful condition, and being able to do that on so many creatures is mind-blowing.

This spell certainly is weird...and cool

i have a 4th level spell called phantasmal killer and it does the EXACT same thing.

Quote from Janderjas >>

i have a 4th level spell called phantasmal killer and it does the EXACT same thing.

 

 

Beyond 20

 

 

 

 

 

 The difference is that Phantasmal Killer only affects one creature, while this effects all creatures in a 30ft radius sphere. Not worth a 9th level slot, but still definitely better than Phantasmal Killer 

Whoa whoa whoa this is op but the same as phantasmall killer

Mechanically speaking, where is the source of their fear? Since they "can't willingly move towards the source" it would matter in some instances. Somewhere within the area of the sphere? Or is it centered at the caster? If it is within the sphere, is it up to the caster to determine where the thing they're afraid of appears to them?

Quote from Darth_Reaver >>

WOTC has really shafted all Illusion spells and the Illusionist wizard so bad. Its on par with the Beastmaster Ranger. Because its so up to interpretation on how illusions work, ive been shafted by every single DM that I've played with when using my Illusionist. 

Look at Psychic Scream, another 9th lvl spell, does more damage and even on a successful save, they still take half damage. This spell? a measly 4d10 and a successful save nothing happens. 

Illusion wizards are by far the most powerful class in the game, if you are creative enough and have a DM that is willing to play ball.
It's really hit or miss based on the DM (similarly to Wild Magic Sorcerer), but in the right group they are truely awesome.
I play an illusion wizard in our campaign now and we just defeated a dragon. Before the fight I cast mirage arcane on the entire dragon lair and area around it. When we engaged the dragon I placed myself in the middle of the lair and cast a wall of force around me. All I did the rest of the fight was to modify the world using Malleable Illusions to restrict the dragons movement, create cover for the others in the party or help the fighter and paladin get in melee with the dragon.

Back on topic, Weird is by far the worst 9th level spell and not even a creative Illusion Wizard with a generous DM should ever use this spell.