So I am looking to homebrew a 7th level sorcerer spell because I am not entirely happy that the options match my characters theme.
I have seen Visions of Danger in Pathfinder 2e and have made an attempted conversion into 5e rules. I am looking for advice on if it is balanced at my first pass.
Visions of Danger: 7th level illusion
Casting time: 1 action
Components: I don't care
Duration: 1 minute (required concentration)
Effects: You create a 20ft radius sphere filled with dangerous looking illusions of your choice at any point you choose within range. The illusion spreads around corners but not through total cover.
When a creature enters the area for the first time, or starts its turn there, the creature must make an Intelligence saving throw. The creature takes 8d8 phychic damage on a failed save and has its speed reduced to 0 as it believes it is grappled by the illusion. On a successful save the creature takes half damage and disbelieves the illusion so will take no additional ill effects on any future turns.
As an action, a creature 'grappled' within the area can attempt another save to disbelieve the illusion. A successful save will end the speed reduction and allow movement as normal as the spell effectively ends for them.
My line of thinking:
There are very few high level spells with ongoing damage effects, so this was a tricky one. I started with Cloudkill (5th level) so that is the lowest level this could be. Scaled up to 7th level Cloudkill would do 7d8 damage, but scaling is never good so I added an extra 1d8.
I then added the extra effect of the speed reduction because I feel that especially at high levels a spell like this needs some way of keeping people in the area.
The ability to disbelieve the illusion is unusual in 5e, as although disbelieving illusions is a thing in the rules I am not aware of it with a damage spell. This probably means there is a lot of additional tracking to do, but my DM will be ok with this and I will help keep track of stuff so I don't take that kind of thing as being a negative as I might if I was creating this to actually publish or if I were answering someone else asking this question.
I have also looked at Incendiary Cloud (8th level) which does 10d8 damage vs 8d8, and heavy obscurement vs grapple. Both Incendiary Cloud and Cloudkill move, but they are area denial spells for their entire duration, unlike this spell.
I therefore think the riders balance out, and the lower damage on this spell means 7th level is about fair.
My only concern is that Incendiary Cloud is (certainly in optimisation circles) a terrible spell, but it is official so I can't exactly clone it, call it lower level and claim it to be balanced.
A bit more reasoning:
Some spells to consider in terms of balance are Phantasmal Force (2nd level), which is where I took the 'use your action to disbelieve' from. The way I play that spell is that I always make the character make the check, and I narrate it as them dealing with the illusion. So if they fail then they swung at the imaginary creature and missed, but if they succeed they realise they hit and it is an illusion. I know the spell doesn't say that exactly, but there are so many arguments over how that spell works I think this is the only fair use.
Also (Evards) Black Tentacles which is similar in that it does damage in an area over a period of time and restricts the ability to escape. That is a 4th level spell doing 3d6 damage. You can't upcast it, but if it followed the usual upcast rules it would do 6d6 at level 7. That is less that this spell, but it includes difficult terrain that I don't have, restrained which is far worse than grappled, and is a check (not a save) to escape. So not only is it harder to save from after the initial save, but the difficult terrain means that most things are going to be stuck in the area for more than one round even if they do save. With this spell one save ends it for them entirely, no being pushed back in, no failing to get out of the area etc.
From a powergaming perspective a spell which remains even after a save is a million miles better, because you can all but ensure the target stays in for the duration even if they make their save all 10 times. This spell has effectively zero synergy with anything other than spells like Bane which make saving throws harder.
Also Maddening Darkness (8th level) was brought up in an answer, but that has more range (not much of a difference), massively larger area, 10 times the duration and the targets can't see (you can't see them either, but you cast the spell presumably when that is a bonus to you). It will take most creatures 2 turns to escape without using an action, but I find it heavily metagaming to assume that a creature knows to make for the nearest (or indeed any) edge in order to escape if they can't see the exit. The spell also lingers, meaning that any good synergy can keep an enemy in the area for a very long time. It does the same damage, but if you cast it in the circumstances it is meant to be used for you will hit way more targets than this spell has any chance of doing.
I think it can seem more powerful than Maddening Darkness, but that is because the situations where Maddening Darkness are truly useful are few and far between, and that isn't a balance issue.
Lastly: Intelligence saves are rarer for monsters to have, but I don't believe that the type of saving throw is accounted for in standard balance equations. If you want to include it in your analysis please do, and I would happily change it to a Wisdom save, but it is worth nothing that I am specifically looking to pigeon hole my character into Intelligence based psychic damage spells, so in a way it is a weakness of mine that I would be building into with this spell. The DM knows this and I know something like a mindflayer dragon is likely just around the corner for example and we have both discussed that I have no issues with their being some fights that are just going to be tough for me mechanically. That doesn't make much difference for standard balance, but if you end up thinking this is level 7.5 then it might tip the balance that for me 7th is appropriate.
Describing the flavour of this spell to explain why I picked these mechanics:
Mostly I will create an army of heavily tentacles eldritch horrors grabbing and attacking, so when a character uses their action to disbelieve the illusion that would be narrated as battling the horror in an attempt to escape. A failed save would mean they missed, or failed to cause damage. A successful save would mean their blow went through and they realised it was an illusion. So it isn't that they just think really hard about it or have to suspect it is an illusion, it just represents them dealing with it. If they do something else with their action, then that is ok but they would have no reason to suspect it is an illusion and doing something else wouldn't give them any clues because they aren't interacting with it physically.