Is there a meaningful difference between giving a monster multiattack and giving it multiple turns per round?

Question

I'm creating a CR21 boss for my medium-level party of six players to fight. I know that action economy is a big disadvantage for a single enemy fighting an entire party, so I'm giving the boss the ability to attack 4 times per turn (20-35 damage per most attacks).

I know the most common way to give a creature multiple attacks is to give it Multiattack, an action that allows it to use its attack abilities several times in one go. I'm considering instead rolling initiative for the boss 4 times and giving it a whole turn for each initiative value, giving it 4 separate attacks.

Will this have a significantly different effect on the battle than giving the boss a standard multiattack?

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A couple thoughts I've had:

• This allows the boss to use more bonus actions per round. The boss I'm designing doesn't have any nonstandard bonus actions, so I don't think this will make a big difference.
• This allows the boss to move more often. I've divided the speed I want the boss to have by the number of turns it gets per round, so its total movement per round will be the same.
• This gives the boss more reactions. I can see this making it quite a but more difficult. I'm not planning on using many reactions when I play the boss, though.

Answer 1

Numerous spells, abilities, and actions change

The boss is stronger in strange ways, because they...

• Can try to save four times per round against hold monster and similar save-or-suck spells. Other effects, like a monk's Stunning Strike, are unaffected.
• Can grapple or shove four times per round. (One cannot substitute a grapple for an attack within Multiattack, unlike Extra Attack.)
• Are less affected by one bad initiative roll. A good initiative roll isn't nearly as important as avoiding being the last creature to act.
• Can stand up from prone four times per round.
• Negate surprise after just one of their four turns in the first round.
• Have four chances to beat the Assassin rogue, thus eliminating the rogue's advantage.
• Can Disengage exactly when they need to, while only using one of their four attacks.
• Can cast four spells if they have spellcasting ability. Typically, casting a one action spell takes the place of all of a creatures attacks.

The boss is weaker in strange ways, because they...

• Are forced to save four times per round or take damage from spirit guardians and similar damage spells.
• Are much more quickly affected by save-X-times-or-suck spells, like contagion and flesh to stone.

Legendary Actions are the normal answer

As the other answers stated, Legendary Actions avoid all of these problems by allowing a Legendary creature to act outside of its own turn. An Adult Red Dragon is one such Legendary creature and has the following Legendary Actions:

Legendary Actions

Can take 3 Legendary Actions, choosing from the options below. Only one legendary action can be used at a time, and only at the end of another creature's turn. Spent legendary actions are regained at the start of each turn.

Detect: The dragon makes a Wisdom (Perception) check.

Tail Attack: The dragon makes a tail Attack.

Wing Attack (Costs 2 Actions): The dragon beats its wings. Each creature within 10 ft. of the dragon must succeed on a DC 22 Dexterity saving throw or take 15 (2d6 + 8) bludgeoning damage and be knocked prone. The dragon can then fly up to half its flying speed.

Legendary Actions can be anything. A high-level Legendary spellcaster could fire off four spells per round. A Legendary demon lord could teleport around the battlefield, striking PCs who thought they were safe. A Legendary archdruid could rapidly switch between different beasts, each one making a different attack.

One more thing - if you don't want your boss to fall to the first spell that the control wizard casts, don't forget to add Legendary Resistances. They may feel cheap when you use them, but allowing the boss to auto-save a couple of times helps the encounter be memorable in the right way. Turn 1 banishment against a creature with 8 CHA shouldn't be the answer to everything...

Answer 2

The D&D 5E system already has built-in rules for important creatures acting out of turn or more than once per turn: Legendary Actions and Lair Actions.

Legendary Actions happen a certain number of times each turn (usually three), though some specific instances take up more than one. These actions take place after another creature's turn, interrupting the initiative order. They range from casting a spell, making attacks, using an ability, extra movement that doesn't provoke attacks of opportunity, to more exotic things.

Lair Actions are similar, but take the form special things the creature can do with the environment (or the environment does to the creature's benefit). They always take place on Initiative Count 20.

If your players know the game (experience with other DMs, or being DMs themselves), they should be comfortable with the concept. Adding your own house rules when there are already mechanics designed in the game just tends to add confusion.

Answer 3

There are more unintended consequences than I could list

Consider that all of the following things are connected to a turn:

• Movement
• Actions
• Bonus Actions
• Reactions
• Object Interactions
• Events that happen automatically during a turn
• Possibly other things I may have forgotten.

If all you need is more attacks, then that means you need to fix everything that is not that attack. Some of the things that need to be fixed seem easy, because they are found on the creature's sheet. For example, to fix the creature's movement you give it less speed (as you yourself plan to do).

However, not everything the creature interacts with is found on its own sheet! How many things exist that interact with movement alone? More than I could count, and every single one has a potential of creating unintended consequences.

For example lets say a creature has a speed of 10 feet so that over the course of its 4 turns it can move 40 feet and someone hits this creature with a ray of frost, which reduces its speed by 10 feet. Now instead of being able to move 30 feet per round, the creature can move 0 feet per round. What do you do now that you have spotted this unintended consequence?
Do you accept that ray of frost is OP against this creature?
Do you create a dedicated ruling for ray of frost?
Do you create a generic ruling for speed reductions (thereby risking new unintended consequences)?

Moreover, some of the unintended consequences can be very nuanced, hard to spot, and/or hard to quantify. For example, a creature with multiple turns can use its strongest at-will attack over and over again, which is boring; whereas the design of the Multiattack action (and legendary actions and lair actions) often forces a creature to use all its attacks, which adds variety even though some of those attacks are weaker.

Now scale up this problem to every feature, spell, ability, trait, magic item, etc that interacts with movement, actions, bonus actions, reactions, object interactions, and/or events that happen automatically during a turn.

You don't have the time to research all these interactions, find the unintended consequences, and devise all the necessary rulings. Therefore, you will have to improvise on the spot when you encounter an unintended consequence, you will have to document those improvised rulings so that you can be consistent the next time, and your players will have to deal with an increasingly large list of improvised rulings.

You should not put yourself and your players through all this trouble when your problem already has well documented solutions:

• Use legendary actions
• Use lair actions
• Use more enemies

Answer 4

Some spells will trigger more often than normal

An interesting consequence of this is any spell that has an effect with the trigger "at the beginning/end of the target's turn" will trigger more often/earlier than otherwise.

Here are two examples:

Wall of Fire

A creature takes the same damage when it enters the wall for the first time on a turn or ends its turn there.

If your creature is unable to escape for whatever reason, they would be taking 4X the intended damage from this spell if they had 4 turns in a round.

It might even be economically viable to have the players in the effect, because they'll only be taking a quarter of the damage!

Shield

Until the start of your next turn, you have a +5 bonus to AC

A spell that would have made life difficult for (up to) a whole round is now only going to last a quarter of the time.

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I'm sure there are many other examples, but those two should get the point across that I'm trying to make.