The Travelers of the Multiverse Unearthed Arcana includes a playable insectoid race called the thri-kreen that has 4 arms, each of which is capable of wielding weapons, albeit with some limitations:
Secondary Arms. You have two slightly smaller secondary arms below your primary pair of arms. The secondary arms function like your primary arms, with the following exceptions:
- You can use a secondary arm to wield a weapon that has the light property, but you can’t use a secondary arm to wield other kinds of weapons.
- You can’t wield a shield with a secondary arm.
For example, a thri-kreen could wield a greatsword with their 2 primary arms and a shortsword with each secondary arm and then make one attack with each sword using Extra Attack and Two-Weapon Fighting, something that would be impossible for a 2-armed character. Still, this specific combo appears to be well-balanced, because the average damage is the same as simply attacking twice with the greatsword: 4d6+2*STR total damage if all attacks hit (and attacking with only the greatsword doesn't even require a bonus action).
However, one could imagine that this might not be true of all weapon combinations.
Is there any combination of weapons that will allow a thri-kreen, with their 4 arms, to deal more average damage than a 2-handed character with the same stats (i.e. same ability scores, same non-racial features, same class levels, etc.)? Or does the action economy prevent things from getting too, er, out of hand?
To narrow the scope of the question, you may assume that the only relevant non-racial feature is Extra Attack, but feel free to go into more detail if your combo depends on a specific class feature or feat.
(Obviously having 4 arms with hands has other benefits, such as being able to manipulate other objects even while holding 2 weapons, or wielding 3 weapons and a shield at once, but for the purposes of this question I'm only asking about damage.)
Reminder: Two Weapon Fighting only triggers from making an attack with a one-handed weapon, even if you take the Dual Wielder feat. So 2 Greatsword attacks + 1 Shortsword attack isn't possible with standard Extra Attack + Bonus Action.
(TWF): When you take the Attack action and attack with a light melee weapon that you’re holding in one hand, you can use a bonus action to attack with a different light melee weapon that you’re holding in the other hand. […]
(Dual Wielder): You can use two-weapon fighting even when the one-handed melee weapons you are wielding aren't light.
Best Answer
With the Fighting Style Two-Weapon Fighting
The Fighting Style Two-Weapon Fighting, available to fighters and rangers, allows you to add your ability score modifier to the damage of the off-hand attack. To then take your example:
With any damage bonus that adds per attack
Again, take your example, where both methods deal 4d6 + 2×STR damage. However, the greatsword-only method does so in two attacks, while the the thri-kreen method does so in three attacks. As such, any damage bonus that adds once per attack give a greater benefit to the thri-kreen method. The game is rife with such bonuses: barbarian's Rage damage bonus, paladin's Improved Divine Smite, paladin's divine favour spell, the enlarge option of the enlarge/reduce spell, warlock's hex spell and ranger's similar hunter's mark spell, etc.
There are also some examples such as paladin's Divine Smite and a battlemaster fighter's damage-boosting manoeuvres that, while more attacks per turn don't give you more damage per short/long rest, they do allow you to unload more of that damage in a single turn, as you can use three smites/manoeuvres per turn, instead of just two.