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:

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 1d10 damage each round (effectively giving you 4d10 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. The damage for a single 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. The downside is that without your concentration slot you could loose out on another Buff for your other party members or DeBuff against an enemy target. And of course, each damage 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.

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 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 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.

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 damage 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 6 creatures in one round.

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.