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?

Mordenkainen's sword isn't underpowered - it's terrible

It's bad for a concentration spell

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 at Wizard or Bard 13, you have tons of concentration spells to chose from.

• First, Mordenkainen's sword for comparison. Deal 33 damage to one target on round one, then 16.5 "for free" on later rounds.


• 
  Reverse gravity is no-save-just-suck for a massive 50' radius. (An enemy that saves is stuck holding on. Melee enemies without flight are screwed.)


• The 6th-level Otto's irresistible dance is no-save-just-dance even for a legendary creature, unless it's immune to charm. With proper timing, this can stop 3 legendary actions and 1 normal action and also burn a legendary resistance.


• The 5th-level wall of force is no-save-just-wait for most Huge or smaller creatures. Unless they have misty step or similar, they wait while you kill their friends.


• The 5th-level Bigby's hand deals 18 damage per round, also with a bonus action. It deals 36 damage per round upcast to 7th-level. It can also grapple, push, and provide cover.

(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.

• 
  Bigby's hand at 5th-level over 3 rounds: 54 damage.


• 
  Mordenkainen's sword at 7th-level over 3 rounds: 66 damage.


• 
  Bigby's hand at 7th-level over 3 rounds: 108 damage (!).

It's bad for a non-concentration spell

In the earlier list, I only ran through concentration spells. What about non-concentration spells?

• 
  Crown of stars from Xanathar's deals 78 damage over 3 rounds - more than the sword and it doesn't require concentration. It also lasts for 1 hour, so you can precast it.


• 
  Forcecage is the evolution of wall of force. No-save-just-wait for most creatures. It even makes teleporting out difficult.


• The 6th-level mass suggestion is save-or-take-the-day-off for 12 creatures not immune to charm.


• The 5th-level steel wind strike from Xanathar's deals 33 damage to 5 creatures in one round.

(Bards can only learn crown of stars and steel wind strike via Magical Secrets.)

Conclusion

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:

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.

A 6th Level Cleric Spell Is Cheaper, And The Cleric Spell Is Arguably Better

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.

In Damage, Mordenkainen's Sword Wins...Sort Of

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.

Mordenkainen's Sword Is More Expensive

For every casting of 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.

Mordenkainen's Sword Takes Longer To Cast

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...

Spiritual Weapon Is Not A Concentration Spell

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.