This is a great start
I always love when player's have a idea for a unique ability they want for their character so even if this was slightly over-powered I would consider allowing it. Overall it looks pretty good and I think you've done a good job and thinking through the balance of the spell overall. I'll just highlight a few things that I noticed from reading it.
"Incandescent light" is ambiguous
While I thematically understand what you were going for. Emitting light is a mechanical term in D&D and "incandescent" isn't one of the types. It would be better to simply stick with "dim light" or "bright light" and specify a distance. Or if you don't intend for it to emit light, consider rewording or removing this entirely.
The area of effect is wrong
A 5 foot cone would only hit the first target. I think the correct area for the effect you want is a 10 foot cone originating from you and directed toward the target of the attack.
Damage is underwhelming
For a second level spell the damage isn't that spectacular. Of the spell you compare it too, Scorching Ray has significantly more range and Dragon's breathe has up to 10 activation per casting. I think you could consider increasing this, consider spells such as Burnings Hands and Ice Knife in your comparison.
No save for additional targets
Basically this is a AoE spell that only requires an attack roll against the closest enemy in the range. If used cleverly you could attack a low AC target and have a high AC target as one of the additional targets which would then take automatic damage. Instead I would consider something like the Ice Knife mechanic where the additional effect is trigger hit or miss but allows a saving throw for half damage.
Missing the attack roll is a big cost
Currently there is a huge opportunity cost to cast this spell. Assuming the fighter is at least level 5 they are giving up their extra attack and a second level spell slot for the chance to do some extra damage. If they miss the attack roll it is all wasted.
I would suggest either activating the effect hit or miss as mentioned above or looking at the Paladin's smite spells that activation on the next melee hit instead of as part of the action to cast it.
Conclusion
I like the spell overall, I think its a good start. And while I think it could be improved I don't think it is so unbalanced that I would ban it at my table. If you do make changes remember that each of the comments I made were to that issue in isolation. Some of the issues are related so if you modify one consider how it effects the others.
Good luck and remember to wait at least 3 days before posted an new iteration to give time for multiple answer to come in.
The attack needs to be mandatory & immediate.
"As part of the same action, you can make a melee or ranged attack..."
So if I choose not to make that attack, the transformed "ally" remains a weapon? New plan: cast charm person on my enemy, convince them to help me fight someone, cast this spell on them, store the transformed weapon in a box, bury the box in the woods, and move on with my life.
Phrasing Suggestions
- Change "you can make a melee or ranged attack" to "you then make a melee or ranged attack". As mentioned above, the attack needs to happen immediately.
- Change "against a creature within 30 ft. of you" to "against a valid target". Let the existing rules for making weapon attacks handle determining range, etc. This means you then need to specify a range for the thrown property, though!
- Change "The damage is appropriate to the form of the weapon" to "You choose whether the damage is piercing, slashing, or bludgeoning when you cast this spell". It's better to specify this directly rather than try to inherit it from the shaped weapon, since you risk people trying to pull in additional weapon properties if those ever get added (whips in particular tend to end up with funky special rules in most editions of D&D).
- Change "within 5 ft of the target of the attack" into "in an unoccupied space as close as possible to the target of the transformed creature's choice". This resolves issues with the target already being surrounded by creatures, and gives your friend a little extra incentive to let you throw them around, since they'll get to choose where they end up.
It feels a little weak if used normally.
This spell has 2 core effects: it lets you make an attack for 1d8 damage with a stat that probably isn't your best one, and it teleports an adjacent ally to the target of your attack.
The teleport effect is potentially nice, but has enough limitations (ally has to be adjacent to you, range limit, not actually a teleport) that it doesn't pull its weight on its own as a level 1 spell.
The damage, in turn, is almost the bare minimum possible. Sure, it's not a d4 or a d6, but the use of Str or Dex rather than your casting stat means you're less likely to hit and get less static damage.
For comparison, consider the 1st level spell catapult from Xanathar's Guide to Everything. It launches an object of your choice (with weight limits) within 60 ft of you up to 90 ft at a target, who must make a Dex save or take 3d8 damage. Compared to that spell, the only thing friend blade has going for it is saving your ally a round of walking.
Remember, any round you cast this spell is a round you didn't cast fire bolt for 1d10 (or more, at higher levels) damage with a 120 ft range.
I would recommend either looking to make this spell more reliable, or make it more of a high-risk/high-reward option (with the ally-teleport as a consolation prize if you miss). You have three dials you can turn to adjust these: whether the attack is a spell attack (using the caster's spellcasting attribute) or a weapon attack (using the better of Str/Dex), how much damage it deals, and what the thrown range is.
For example, you could leave it as a weapon attack, up the damage, to 2d12 or so, and give the thrown modifier a range of (15/60). That means the caster can potentially move their ally a good distance, with the added bonus of a low-but-not-zero chance of doing some good damage.
Alternately, you might change it to a spell attack, bump the damage to 1d10 or 2d6, and give the thrown modifier a range of (45/45). That gets you fire bolt damage, trading a bit of range (which isn't needed in most encounters) for a useful effect of moving your ally farther than they could likely go in one round without Dashing.
Best Answer
This is likely over the top
First, note that in spite of the name, the way this is written the caster can hurl themselves, as they can be "a willing creature", so this is not limited to friends.
In summary, I think this is comparable in power to Dimension Door, with Fireball added in on top, which is too much for a level four spell. It also is clearly stronger than alternative options that transport others or transport and deal damage scaled to 4th level.
Compare to Dimension Door
The main spell that comes to mind here as a comparison is Dimension Door, likewise a 4th level spell with exactly the same radius. This has several advantages over Dimension Door:
It also has one downside:
I personally think that the downside is larger than any of the upsides, but there are multiple upsides, so it is not really clear which of the two is the better spell. And Dimension Door is a top-notch spell already. This is before any damage. Without the damage, I think it could be balanced.
Compare to Fireball
Now, on top you add damage.
The second spell that comes to mind here is Fireball, with the same 20' radius blast, only now it has a 500' range instead of a 150' range. This is huge, essentially no other area damage of similar level has such a range, even Freezing Sphere or Ice Storm only have 300'. Basic Fireball would do 8d6 Fire, which monsters on average have 10% resistance to, and on average 60% fail their Dex save. This does 5d6 Thunder, which on average only 1% are resistant to, and only about 54% fail their Con save. So effectively this is only slightly weaker on the damage front than Fireball against creatures, and clearly better against Objects, where it deals 10d6 immediately, and it has a huge range.
In addition this has the big benefit of when catapulting your melee basher into a group of opponents, you kill off all the weaker ones and remove whatever pack tactics, help actions or additional attacks and opportunity attacks they could provide, leaving only the damaged boss opponents to deal with.
I think adding effectively a free Fireball on top of Dimension Door is too much. It also economizes on preparation slots, as these are normally two must-have picks, that consume two of your precious slots, and now you can cover both needs with one.
Compare to Thunder Step and Vortex Warp
A comparison that combines transport and area damage is Thunder Step from XGtE. Cast at fourth level, this only has a range of 90 feet, only works to a spot you can see (so you nearly always would be able to throw in a straight line there), only works on the caster themselves, only can bring along one Medium creature like Dimension Door. It deals 4d10, comparable to your spell, but only in 10 foot radius around the area you leave (which is much less useful, IMO, as you cannot use it as a faux Fireball from the back rank). This would indicate that to balance this, your radius should at least be reduced to 10', and your range should be cut to 90 feet, and the scaling should go, and then this still would likely be more useful and flexible.
Compared to Vortex Warp from Strixhaven for transporting others, upcast at 4th level1: this transports another creature (including unwilling, but only onto surface, as you have it -- you could use the language from there) 150 feet to an unoccupied space you can see. So this is essentially your spell minus being able to transport yourself, minus 250 foot range, minus scaling to multiple targets, minus any damage, with sight requirement, all in exchange for waiving willingness. I think it is clear that your version is overpowered in comparison. Scatter, from XGtE can do it for 5 creatures, but only 120 feet and is 6th level. In fact, either is so close in what it does at its core, if you wanted a balanced version you could just use it and rename it "Catapult Creature".
Wording
There are some wording issues:
I would recommend to either entirely remove the damage, or significantly reduce the amount and/or radius of the damage, to reduce the range, and to remove the ability to throw multiple creatures.
1 A second level spell that can be upcast to fourth, generally is preferable as you can pick it earlier and so you get more use out of it in a real campaign where you level up from first level. But the goal here is to compare their power at fourth level.