Mryntu

Mordenkainen's Sword seems to be under powered for a 7th level spell, it only does 3d10 damage and 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 it's level.
So is it just me or is it not a very good spell?

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 than, 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 specked into appropriate cantrips, you can easily get another 1d10 damage each round (effectively giving you 4d10 potential damage starting round 2). This plus an eldritch blast on a Bard, gives you a consistent potential 6d10 damage every round of FORCE damage every round for 10 rounds. Giving an average of 330 damage (Max 660) damage for the cost of a concentration slot, your bonus action and using 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. 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. The drawback is that the spell takes 10 rounds to complete and you can't cast another concentration requiring spell. 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 you options and utility towards the party and while in combat.