What would the 5e phrasing be for combining multiple attacks into a single attack?

Question

A (homebrew) magic greatsword has this feature:

Heavy Blow. When you take the Attack action on your turn, you may choose to forfeit your attacks for one very powerful attack. The powerful attack uses the lowest modifier in the attacks you forfeit. If you hit with this very powerful attack, you deal damage as if you had hit every attack you forfeit, and [...]

The intended behavior is that an 11th level fighter with a +8 to hit would be able to use his action to make a single attack with a +8 to hit, dealing 6d6+12 damage on a hit (2d6*3+STR*3).

If this fighter has great weapon master, then the attack would be a +3 to hit to deal 6d6+42 damage on a hit (2d6*3+STR*3+10*3).

How would an effect like this be implemented to most accurately follow the design conventions of 5e?

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Related question (the feature after the and): Is there a way to stun a creature without using a Spell Save DC?

Answer 1

The key phrase is "extra attack", and you need a couple other changes

It's "Extra Attack" that you're talking about:

Extra Attack

Beginning at 5th level, you can attack twice, instead of once, whenever you take the Attack action on your turn.

The number of attacks increases to three when you reach 11th level in this class and to four when you reach 20th level in this class.

And you wouldn't "forfeit" extra attack in your Attack action. You would use an Action to activate the Heavy Blow feature of your magic item (great sword), and say something like:

Heavy Blow

You can use an action to make one melee attack which, on a hit, does damage equal to the total damage you would make if you hit with this weapon using all attacks in your Extra Attack feature (if you have that feature).

The reasons I think this follows the writing style convention better are (1) magic items in 5e commonly say that "You can use an action to..." do something whereas (2) there is not really a model that I know of in 5e for a magic item having you "forfeit" your extra attacks.

From the wording I have proposed, the following should follow deductively:

• If you use the "-5 for a +10" feature from Great Weapon Master with Heavy Blow, and you hit, the damage is the same as if you had hit with all your extra attacks using that same GWM feature.
• If you are a Paladin, you can add the damage of at most one Branding Smite to Heavy Blow (using up a bonus action and a spell slot to do so), because that's the most you could do using your Extra Attack feature.

Answer 2

This appears to be an item designed for 3rd edition of D&D where the attack bonus and number of secondary attacks depended on your primary attack's base attack bonus. Under that system, you would have the choice to give up your best attack for a chance to do double damage on your second best attack, or give up your two best attacks for a chance to do triple damage on your third best attack and so on.

Multiple attacks in 5th edition don't work that way at all so there is no easy or direct port.

The best way to port these is to capture the essence of the item and look at things that do that sort of thing. Like the second point of Great Weapon Master:

Before you make a melee attack with a heavy weapon that you are proficient with, you can choose to take a -5 penalty to the attack roll. If the attack hits, you add +10 to the attack's damage.

You could use this directly once you decide how it interacts with somebody who actually has this feat.

Answer 3

Here is how I would word it:

Heavy Blow.

When you take the Attack action on your turn to attack with this weapon, if you have the Extra Attack feature, you can choose to forgo any extra attacks you could make with this weapon as part of that action and instead add the damage of those attacks to the first attack you make with that action. You must choose to do so before rolling your first attack. You cannot do so if any of the attacks you forgo would have a lower attack modifier than the first attack. If you forgo at least one attack, [additional effect happens].

I believe this implements what you are looking for: it combines all the damage into a single attack, and it forces you to use the lowest attack modifier from among all the attacks that you combined. It should interact with GWM damage rolls in the way you described, since you can declare that all attacks would be made using GWM and then forgo those extra attacks to add their damage to the first one. It doesn't unintentionally multiply the effects of modifiers to a single attack's damage, since it is just adding the damage of multiple attacks, not multiplying.

I've tried to cover all the edge cases I can think of with this wording. For example, if a character with Extra Attack is affected by a slow spell, they still have the Extra Attack feature, but can only make a single attack with their action. The last sentence prevents them from gaining any benefit from this feature in that case. My wording also requires that the attack you roll and the attacks you forgo must all use this weapon, so there's no way to attack once with this weapon while forgoing extra attacks from a more powerful weapon for extra damage.

Answer 4

My suggestion:

When you take the Attack action on your turn and attack with [this weapon], you can choose to forgo all but one of the attacks made as part of the action. You must choose to do so before the attack roll of the first attack made as part of this action. If you do so and then hit with that attack, roll all of the attack's weapon damage dice an additional number of times equal to the number of forgone attacks and add them together. Then add any relevant modifiers this additional number of times as well.

My reasoning for the phrasing is as follows:

The beginning is taken from a combination of two-weapon fighting:

When you take the Attack action and attack with a light melee weapon that you're holding in one hand, you can[...]

...and the Battlemaster fighter's Commander's Strike maneuver:

When you take the Attack action on your turn, you can forgo one of your attacks[...]

The "You must choose to do so before the attack roll" bit ensures that this choice is made before the attack roll is/any extra attacks are, and comes roughly from the wizard's Portent feature:

You must choose to do so before the roll[...]

The phrasing of how to roll additional dice and add it/modifiers to the result is based on the critical strike rules, modifying it for rolling and adding more than just 'twice':

Roll all of the attack's damage dice twice and add them together. Then add any relevant modifiers as normal.

The phrasing for "an additional number of times equal to the number of forgone attacks" doesn't really have a base I could find (and, honestly, feels slightly off-spec to me), but ensures there's no trickery such as benefiting from heavy blow on a haste'd Attack action.

Note that compared to the critical strike rules, I added very specifically weapon damage dice (à la the XGtE Orcish Fury feat) to avoid things like freely doubling Divine Smite damage... or at least I'm pretty sure that tweak blocks that, since Divine Smite damage is considered "in addition to the weapon’s damage" rather than 'additional weapon damage'.

As @Ryan Thompson notes, this allows any features that apply an extra one-time flat modifier to be multiplied, such as the goblin's Fury of the Small. If that is a concern, you could modify the last sentence to be along the lines of:

Then add any relevant modifiers this additional number of times as well, unless the modifier is limited in how often it can be applied.

That's getting into the territory of being out of 5e's style, and may require more frequent individual rulings.

Answer 5

You do not "forfeit attacks" in 5e. You can forgo a single attack as part of an attack action in return for specific attacks (commander's strike), but using your action to activate a magical effect instead of making an attack action is not "forfeiting your attack". By bringing attention to the fact that, with this weapon you can use your action to make an item effect but can't attack as well, it leads to confusion because it implies that one can normally use an action for an item and attack. It's why this kind of phrasing isn't used in 5e design convention.

Additionally, your original phrasing leaves a lot of wiggle room for overpowered combinations, and needs to specify what benefits can apply as if it's a melee attack and what benefits do not, and needs to specify that this is not an attack action. Take apart this phrasing and use what you like.

"While wielding this greatsword, you may use your action to deliver a heavy blow with the greatsword. Make a greatsword melee attack. If it hits, calculate damage as if you used your action to attack with a greatsword the maximum times you are able as an attack action.

You may apply modifiers from class features, racial features, spell effects, and character feats to the initial attack roll and subsequent damage calculations as if making a melee weapon attack, but only if they could reasonably apply to all attacks on your turn. Limited use features like divine smite, single-opportunity advantage, or true strike cannot be applied to a heavy blow action."