I appreciate the support the spell is getting. And I always appreciate Aaron's feedback, as it's usually well thought out.
I'm not usually got on the math/mechanical side of things when it comes to creating spells or such.
My observations and comments, in as orderly a fashion as I can manage:
"You can use your attack action using your spell attack to make one attack with each Spear of Obsidian to a range of 20 feet." should be worded more in-line with what language existing spells use, especially because use of the Attack action implies that a multi-class character with Extra Attack actually gets additional attacks and likely a few other interactions the design of the game aims to avoid because they unbalance things.
Something like "You can use your action to make a melee spell attack with each Spear of Obsidian, which have a reach of 20 feet." if you intend these attacks to be melee attacks, or "You can use your action to make a ranged spell attack with each Spear of Obsidian at a range of 20 feet." if you intend these attacks to be ranged attacks.
Yes, I intend them to be melee attacks. I will change the wording on the spell.
As for the damage, as written it is above the recommended damage for a 3rd level spell - so either that should come down significantly, or the level of the spell should be raised (there are guidelines on page 283 of the DMG).
I am looking at the DMG right now, for one target it states 5d10 damage, which is an average of 27.5 damage to the singular target.
Or, multiple targets which is 6d6, which is an average of 21 damage per target in X area.
Now, when referring to multiple targets, does it refer to AoE spells, spells that roll separate attack rolls for each attack (such as this), or both?
The way I was looking at it, the spell would fall somewhere in-between since each attack is a separate roll.
Additionally, the spell shouldn't be dealing as much damage as other spells of its same level and also providing defensive benefits.
Also, the scaling AC bonus: anything higher than a +2 makes this spell better at defense than most of the AC boosting spells, while it still gets offensive potential.
Perhaps the spell should grant +1 Armor Class per every 3 spell levels? So it's a max of +3 Armor Class if you cast it as a 9th level spell?
There is also the issue of the reactions to "block" attacks and spells: firstly that isn't a term that is used in the existing rules so it sticks out as undefined, and secondly that what you have done is almost like making a 3rd level spell that does the best damage of any 3rd level spells, gives more long-term AC than shield of faith, and gives you a free counterspell and shield in effect - so it's sort of the power of two 1st level spells, a 3rd level spell, and also a bunch of damage.
I was aiming to give the caster a bit more utility. I figured everyone gets one possible Reaction a turn. They would be using the limited resources of their legs to negate attacks. Perhaps they should lose 2 legs per melee attack negated this way? And still lose a number of legs equal to attack spell cast at them? Or should I have it so they can just use their reaction to block Cantrips at the cost of 2 legs? Ray attacks that shoot more than one ray would allow the caster to only negate one ray, while the opposing caster could still him their target with the other ray attacks.
Here is my thought process on why this spell could work, with a few minor tweaks from feedback:
1. Lose the bonus damage from caster's casting stat modifier.
2. Lower Armor Class gain to +1 every 3 spell levels, to a max of +3 Armor Class as a 9th level spell.
3. Make the spell require an Obsidian Gem worth at least 50GP and the drop of Ichor, which are consumed.
4. Allow the use of 2 legs on a Reaction to negate one melee attack or one Cantrip attack, shattering the legs after the Reaction.
Here is my thought process on how combat would play out:
Round 1
Caster casts Spears of Obsidian, only has Move Action and Bonus Action left. Gains option of "negate" one melee attack or Cantrip upon a reaction.
Opposing caster casts Magic Missile on the Obsidian Spears, wiping out 2 of them, leaving only 2 for the caster to possibly use on their next turn.
Fighter runs up to caster, using two attacks to attack the obsidian spears, breaking them, leaving 0 for the caster to use on their turn.
or
Opposing caster casts Fireball, both the caster and the Spears of Obsidian need to make a save. Fireball wipes out all or most of the spears. Caster on their next turn has 0-1 spears to use.
One could argue the spears could potentially draw attacks away from the caster, or give the caster more HP. But they are so fragile and there are some many ways around them, such as melee attacks, ranged attacks, AoE spells (which is the big one), direct damage attacks such as Magic Missile (which the spears are not immune to), etc...
A caster has the lowest Hit-Die, assuming only the d6 caster's could use it. So let's say a caster who can cast 9th level spells casts this spell, it grants them 10 Spears of Obsidian from out of their back. Their Proficiency Bonus is a +6, their casting stat we'll say is a 20, granting a +5, so their attack with each spear should be a +11 to a d20 roll.
Each of the 10 spears would have to roll a successful attack, and if they do hit it's only 2d4 damage if the spell's damage is modified, or 2d4+5 if it is not. So an average of 5 damage a successful hit, to an average of 10 damage a successful hit. This is also not including the round that must cycle before the caster could even use their Spears of Obsidian, so it's possible several of their spears may be broken by the time their turn comes up again. When instead a caster could do more reliable damage with a well placed fireball as a 9th level spell which is 14d6, or 49 average damage to those who fail their save, to 24.5 average damage to those who pass their save, in a 20' radius. That is 64 potential targets, vs 10 potential targets. Guaranteed damage (unless there are things such as perks negating damage or making you immune to fire) vs damage one must roll for.
Mage Armor sets an Arcane Caster's Armor Class to 13 at level 1, and has an 8 hour duration, vs a 1 minute duration.
Shield of Faith is a +2 Armor Class, but it's duration is 10 minutes, vs a 1 minute duration.
Though a 2nd level spell, Barkskin doesn't allow the recipients Armor Class to drop below 16, it's duration is 1 hour.
Counterspell effects those within 60 feet and you must use a spell slot of equal level. You can also attempt to stop a spell of higher level than what you cast by making an ability check. The Reaction to Spears of Obsidian would not only require you to cast a spell of similar level, but also require it to be an attack spell against you, which you must sacrifice x amount of Spears of Obsidian to negate the one attack, instead of stopping a whole spell overall.
So, all-in-all do I find this spell overpowered? After looking over everything, not really. I feel it has a lot of utility potential, doing a lot of little things compared to spells that either do it at greater effect or for a longer duration. The only thing that I saw that could make it potentially overpowered is the caster's casting stat modifier added to the damage rolls, that combined with an attack for each spear. But then again, I don't see any feats/abilities, or even weapon Enchantments that the caster could get that would enhance the spears, vs a fighter for example that could have a +3 sword with the duelist feat, and use their Action Surge (which is a limited well of power just as this spell would be), for an effective 1d8, +2, +3, +5, for 5 attacks.
I'll make some edits, however.
