The Mordenkainen's Sword spell seems to be under-powered for a 7th-level spell. It only does 3d10 damage; while admittedly it can do this damage every round for 1 minute, it still just seems to not be that much damage for a spell of its level.
Is it just me or is it not a very good spell, compared to other 7th-level spells?
At any given level, the spell you chose for concentration should be one designed to "win" that encounter. When you first gain 7th-level spells as a 13th level wizard or bard, you have tons of concentration spells to chose from.
(Bards can only learn reverse gravity, wall of force, and Bigby's hand via Magical Secrets.)
I think it's important to highlight how much better Bigby's hand is than Mordenkainen's sword. In my experience, most fights in 5e last around 3 rounds. For this calculation, I will assume all attacks hit.
In the earlier list, I only ran through concentration spells. What about non-concentration spells?
(Bards can only learn crown of stars and steel wind strike via Magical Secrets.)
Mordenkainen's sword isn't just underpowered, it's brokenly bad. In nearly all situations, I would rather spend concentration on a 3rd-level hypnotic pattern than Mordenkainen's sword. The sword would be underpowered as a 5th-level spell, much less 7th.
Want to win most encounters? Pick a control spell. Want to deal a bunch of damage? Pick Bigby's hand or steel wind strike. Whatever you do, don't pick Mordenkainen's sword.
Mordenkainen's Sword is comparable to the 2nd level Cleric spell Spiritual Weapon. It has the same range, lasts for the same amount of time, and you are allowed to move and attack with it as a bonus action every round.
Cast as a 6th level spell (it scales with every 2 levels above 2nd) Spiritual Weapon deals 3d8+SpellCasting Ability Modifier Damage, compared to Mordenkainen's sword, which does 3d10. Both are force damage, and Mordenkainen's sword has a higher maximum damage potential...unless the Spellcaster has a +6 as their modifier. However, Mordenkainen's sword also has a lower damage minimum - 3, compared to 3+Ability Modifier.
This is the only edge that Mordenkainen's Sword has.
To cast Mordenkainen's sword, you must first purchase a 250gp miniature platinum sword. This item is not consumed, but is a required purchase before you can cast the spell. Spiritual Weapon has no material components at all.
The Spiritual Weapon spell is actually a bonus action, which means you can cast it alongside a cantrip on the same round. Mordenkainen's Sword is an Action to cast, meaning any other spell you cast must be a Bonus Action.
But by far the worst part about Mordenkainen's Sword, compared to Spiritual Weapon...
A Cleric who casts Spiritual Weapon does not have to concentrate on it to keep attacking with it - it just keeps going on its own, and leaves you open to cast any concentration spell you like.
Ultimately, direct comparisons between damage output and effectiveness aren't going to be perfect between Arcane and Divine casters. But the list of benefits for Mordenkainen's Sword, a level above what you'd be casting Spiritual Weapon at, are so small compared to the detriments...
You'd be better off playing a Cleric.
Mordenkainen's Sword:
This spell can do up to 3d10 x2 damage on the first round. First you use your action to summon it, and it procs, then you can use your bonus action to move it 20 ft and attack. Then each round after that, it can make an additional attack. Maximum damage for a single casting is 33d10 (10 rounds, plus the summoning proc). It requires concentration, so consider it "Bonus" damage at the expense of your Bonus Action. Meaning you can still take your ACTION to cast other spells.
This gives you a potential Average damage of 181.5 damage over the course of the spell with a maximum of 330 damage, for a single spell slot. Making it exceedingly efficient, but very slow. If you have a basic damage cantrip, you can easily get another 3d10 damage each round (effectively giving you 6d10 potential damage starting round 2). If you have access to eldritch blast (Bard), this gives you a consistent potential 6d10 of FORCE damage every round for 10 rounds. Giving an average of 330 damage (Max 600) for the cost of 7th level slot, a concentration slot, your bonus action and using your action on a cantrip every round.
The high damage output plus the fact that it's force damage makes it a threat IF you party needs heavy damage output and can tank for you. Getting hit and loosing concentration can severely mitigate it's effectiveness. The tradeoff comes from the idea that very few things have Force resistance, and the spell uses your attack roll rather than the targets saving throw. The damage for this level 7 spell slot is above average compared to similar spells of that level. Another drawback is that the spell takes 10 rounds to complete and you can't cast another concentration requiring spell. Utilizing other spells or abilities that require a bonus action will also hamper damage output. Though you can still cast other instantaneous spells (like fireball) with your regular action.
It's heavy damage over time and does not require a class level dip or expenditure of Magical Secretes (Bard) to acquire. The downside is that without your concentration slot you could loose out on a Buff for your other party members or DeBuff against an enemy target. And of course, each damage proc requires an attack roll.
In conclusion - If you need additional damage pressure against a creature with heavy resistances, this is VERY cost effective for the damage dealt. But the length of time it takes to reach max damage potential at the cost of concentration / bonus action seriously hampers your options and utility for the party while in combat.
As explained in the other answers, Mordenkainen's Sword very clearly falls short of both an upcast Spiritual Weapon or an upcast Bigby's Hand (doubly insulting, considering Bigby was Mordenkainens henchman and apprentice).
Spiritual Weapon can be expected to do comparable damage using a spell slot one level lower, and not hogging your concentration (and you can use lower level spell slots with it, making it more flexible, too), and Bigby's Hand does over 60% more damage if upcast to the same level, and has a lot of other utility, too. All of them deal force damage, so there is no advantage in that.
Considering the damage output from Bigby's Hand at 5th level for a typcial fight, a power-adjusted level for Mordenkainen's Sword would likly be around 5th level -- its damage is slightly higher, but it lacks the versatility of the hand. So, why is it on 7th level instead? Or why does it not deal more damage?
I think the first question has it answer in history. Mordenkainen's Sword has been around since the first edition of the PHB, and back then was put in 7th level. During that time, it did 5-20 damage (5d4, average 12.5) to medium or smaller and 5-30 damage (5d6, average 17.5) to larger creatures, and hit points were a lot lower back then. For example a Hill Giant had an average of 37.5, so with a bit of luck you could kill one with just two hits. Compare this to the 5e sword, where the Hill Giant has 105 hit points, and you need about six hits to kill them, three times as many. So, historically, the spell could be said to have done two or three times as much effective damage, and if it still did, it would be on par or better with an upcast Bigby's Hand.
In the original version, there were also monsters that could only be hit by +3 or better weapons, and like in today's game, many of them had resistances or immunities against damage types like cold or fire. So sword could hit them, and therefore was an important tool to ensure damage got through, just like magic missile. Level seven was appropriate. And while the designers did shift the levels of some spells (for example, Bigby's Hand was a whole collection of spells spread over many levels, and still reflected in today's modes), the default is to have them at the same level as they always were.
Why then does the new sword deal so little damage, why not 5d10 or 6d10? I do not have a direct answer to that.
However, I think that there is not only downside in not entirely balancing all spells, in having some signature spells like fireball, that are slightly too good for their level, and others, like this one, that are slightly (or even significantly) too weak.
Firstly, how important optimization is depends very much on play style. In the end, the DMs task is to present his players with exciting, adequate and challenging encounters, but not to overwhelm them. If a party happens to select a lot of sub-optimal or weak spells or character options, and as a result is weaker than a more optimized one, that does not have to be a bad thing. The DM can just tone down the power level or number of the opponents. In that context, if your character concept is to have a wizard that can fight with a cool, magical floating sword, Mordenkainen's Sword is the perfect spell for you. The spell has great flavor.
Secondly, even for players who like to optimize, having stronger and weaker spells is a benefit. If everything was perfectly balanced, there would nothing to be optimized, and nothing to be gained from mastery of the game system and from understanding what is powerful and what is not. Having stronger and weaker spells allows players to discover the internal structure of the game, and be rewarded by being able to build stronger characters. Once you realize that Mordenkainen's Sword is a joke for a 7th level spell, and Simulacrum is entirely over the top, you can make use of it in your spell selection.
