Phantasmal killer (4th-level) and weird (9th-level) have the same effect, with the only differences being the level and that weird affects all creatures in a 30 ft sphere, and phantasmal killer affects a single target:
Phantasmal Killer
4th-level illusion
You tap into the nightmares of a creature you can see within range and create an illusory manifestation of its deepest fears, visible only to that creature. The target must make a Wisdom saving throw. On a failed save, the target becomes frightened for the duration. At the end of each of the target's turns before the spell ends, the target must succeed on a Wisdom saving throw or take 4d10 psychic damage. On a successful save, the spell ends.
Weird
9th-level illusion
Drawing on the deepest fears of a group of creatures, you create illusory creatures in their minds, visible only to them. Each creature in a 30-foot-radius sphere centered on a point of your choice within range must make a Wisdom saving throw. On a failed save, a creature becomes frightened for the duration. The illusion calls on the creature's deepest fears, manifesting its worst nightmares as an implacable threat. At the end of each of the frightened creature's turns, it must succeed on a Wisdom saving throw or take 4d10 psychic damage. On a successful save, the spell ends for that creature.
As we often do with homebrew spell reviews asking "what level should this homebrew spell be?", I'd like to see an analysis of weird in relation to phantasmal killer and other spells. To me, it seems grossly underpowered compared to other 9th-level spells, with psychic scream being a particularly apt comparison:
Psychic Scream
9th-level enchantment
You unleash the power of your mind to blast the intellect of up to ten creatures of your choice that you can see within range. Creatures that have an Intelligence score of 2 or lower are unaffected.
Each target must make an Intelligence saving throw. On a failed save, a target takes 14d6 psychic damage and is stunned. On a successful save, a target takes half as much damage and isn’t stunned. If a target is killed by this damage, its head explodes, assuming it has one.
A stunned target can make an Intelligence saving throw at the end of each of its turns. On a successful save, the stunning effect ends.
Stunned is much more powerful than frightened (in my estimation), and it takes three saving throw failures against weird to approach the damage of one saving throw failure against psychic scream (8d10 ≈ 44 for three failure to weird, 14d6 ≈ 49 for one failures to psychic scream, which also deals half on a successful save), and psychic scream affects a larger area.
Maybe I'm missing something about the utility of frightening a large group of creatures, but weird just seems like complete trash for a 9th level slot. Is adding the area of effect to phantasmal killer really enough to take it from 4th to 9th level? And if not, what level should weird really be, to be balanced relative to other spells?
Best Answer
You're not missing anything - phantasmal killer and weird both suck
Both of these spells are very expensive for what they do, and probabyly both of them are underpowered for the level they are on.
Phantasmal Killer
Let's start with the base effect, phantasmal killer. For a 4th level slot, you get to frighten a single creature. Compare that with fear, an only 3rd level spell, that may have a lower range (30 feet instead of 120), but with a cone can affect multiple creatures. Just looking at the fear aspect, phantasmal killer would be arguably weaker than a spell that is one level lower.
Lets look at the damage aspect: if the target fails two saves in a row against Wisdom, it will take 4d10 damage. What is the expected damage here? Wisdom saves have an average failure chance of an expected 61% up to level 10 CR for level-adequate foes. This means that the expected damage at the end of the first round is about 8 points. Even if you look at overall expected damage over many more rounds, the total would not exceed 21, and over the typical 3-4 rounds of a fight, it would be 18. Compare this to the expected 22 points after saves for a simple fireball spell, that is delivered immediately again can hit multiple targets, is one level lower with level three (and does even have a larger range). This sucks big time, as a damage spells. (Fireball has a worse damage type, but the difference is so stark here, with fireball easily hitting 4 or more targets that this is really irrelevant).
So, it the combination of the two maybe great? Not really. It is better to have a clean, cheaply priced specific effect, than a costly bag of several half-baked effects none of which does its part well. The fear aspect may hold off a powerful foe for a round, and with a lot of luck even for 2-3 rounds. But for such a foe, the paltry damage will pose little impact. You get CR4 opponents at caster level 7. At that challenge rating, the average Monster Manual monster has 117 hit points (168 if you follow the guidance for building monsters in the DMG). Losing 8 or 16 points of that just doesn't make a big difference. For a single target, and a level 4 slot, that is a woefully weak effect. You could take out that foe entirely without resaves with banishment or polymorph on that level, outside the paltry damage add-ons.
Weird
Your question is about an adequate level of weird. Again, the fear aspect is pretty comparable to the fear spell, but it has a larger range, and with a full 30-foot radius sphere, also a four times larger area than the 30-foot cone. But you can also compare this to the hypnotic pattern spell. This only affects a 30-foot cube, and instead of about 2,800 sqare feet, only 900 square feet or about a third of the area. However, with hypnotic pattern, the creature is fully incapacitated -- it cannot cast spells or take other actions a frightened creature can, and it does not get resaves. That is much stronger. I'd rather have the hypnotic pattern at level 3, than the fear aspect of weird, even if weird was level 3, not level 9. There is the occasional fight where the greater radius of weird will be of benefit, but in many fights in narrow dungeon environments, there is little effective difference between the usefulness of the 30-foot cube and the 30-foot radius sphere.
For damage, do we really need to go there: the benchmark of fireball covers a smaller area of about 1250 square feet, a bit less than half what weird does, but it deals more damage, faster. If you upgrade to a level 6 circle of death, you get a larger area of 60 feet radius, larger range, and better damage (now of the good necrotic type). So the damage aspect is maybe appropriate for level 4 here, and clearly weaker than level 6. (I won't even compare to meteor storm, the damage difference is obviously ridiculous).
You get access to level 9 spells in tier 4 at level 17. Again, for such opponents, who can be expected to have over 250 hit points (Monster Manual) or over 318 (DMG), the damage aspect does next to nothing, and you are mostly banking on holding them off for a turn or two.
Even if you added the spell levels of fireball and fear linearily, the level of this overall effect would be no higher than level 6. That is not a bad take on where the spell would probably be more interesting. I still would not pick it, because in my experience, spells that cannot decide what they want to do well don't work well. If I had to re-level these spells for my home game, I would put phantasmal killer on spell level three, and weird on spell level five.