Flurry of Strikes
Casting Time: 1 action
Range: Self
Components: V, M (a melee weapon)
Duration: Instant
Effect: For each opponent you can see within 60 feet, you teleport to an unoccupied space adjacent to that opponent and make a melee weapon attack. Then you can teleport up to 60 feet to an unoccupied space you can see.
Writing first before reading so I give you an straight idea, but since people probably have thought about something I missed this probably isn't complete unless you think about everyone's response.
Well, dang. 60' radius is quite big, so the variation on this is hard to tell. Same with the damage - a melee weapon attack can have a lot of riders on it. Now, the fact that it's self-only has some limits, and you can't be a raging barbarian using reckless attack. But you likely have a decent melee weapon attack to know/prepare this spell, so thinking of something like a fighter (samurai) in your advantage-to-all-attacks is not out of the question.
With say a flaming weapon that adds and extra 2d6 damage to every attack. Or someone with the shadowblade spell doing 2d8, 3d8, 4d8 damage each attack (with possible advantage) depending on how far they upcast it. That's almost likely as good spells to have together. Holy Weapon and various other things will improve every weapon strike.
There are also times that can do other things, but I'm not finding good examples. Can't find a "prone every attack", fighter (cavalier) can mark everyone but there's no effect if they don't end up within 5'. Vampiric Touch is a melee Spell attack so it can't be used. Okay, let me not borrow trouble.
So, do I look at this from a maximal targets & maximal damage perspective, which will be correct when "properly abused", or for a more conservative look? Well, I don't think the spell will be picked to be cast when it would have below average returns, so I will look at it with this is the right spell to use a slot on - good amounts of targets and good damage.
So let's say a chance to hit about the same as a non-trained save. Magical (assumed) P/B/S is actually fairly little resisted (thanks to a great ENworld chart where they worked out all of the resistances). So the damage is a little more valuable.
Conjure Volley is a 5th level spell with only a 40' radius, but it does non-magical 8d8 (36 average) P/B/S vs. a Dex save for half. This is half again the radius, which is a bit more than three times the volume. But unless you're looking at wiping out an army of bugbears, there's usually an upper limit to the number of targets. This spell does somewhat less damage, and while a hit is more likely then a failed Dex save, a miss is 0 vs. half. From those we'd be looking more positives then negative, so estimating 1 level higher. But this is a selective AoE, Conjure Volley is everyone. Add another level for that. That makes 7th.
At 7th level, I think throwing in a double strength misty step for the ending teleport for free is just fine.
But ... it doesn't seem to pass the sanity check. Fire Storm, a somewhat selective 7th level damage spell, seems to be pretty much superior to it. From this I think my rough calculations of conjure volley, with a superior damage amount with save for half vs. 3x the volume and landing a hit easier then failing a dex save, and selective, might have been off. If we assume those are even, give +1 for selective targeting we get to 6th. That seems more reasonable.
It's definitely a "many actions worth of effect" for a 11th level martial character condensed in one action.
As a side note, this spell screams out to me to be lower level and upcast to increase the radius.
You know, I wouldn't be as worried about that huge volume if we could eliminate the corner case of slicing down a huge chunk of an army or something. If there was a cap to the number of attacks, high enough you'd never hit it except in that corner case (as 12 or 15?) I'd feel better.
BTW, for all that I'm not sure I'd allow this spell at my table, but not for balance reasons. It's simply a lot of rolling. A single attack roll and damage roll might be too out of whack - a crit or 1 could vastly change the value to the spell, but even an abnormally high or low to hit roll would have an outsides effect positive or negative. Maybe a single damage roll. Not sure, but I'd definitely want to speed it up, since it's at it's slowest when it's most likely to be cast - when there are a lot of opponents nearby.