Can someone use a Readied Help action between multiple attacks?

Question

• Scenario 1 : An ally is unseen and has Extra Attack. They have advantage on the first attack of their Attack action and I want to use the Help action to give them advantage on the second attack of their Attack action.
• Scenario 2 : I am a level 5+ Warlock. I have a familiar, I am unseen and want to get advantage on both beams, so I want my familiar to use the Help action for the second beam.

Is it possible to Ready the Help action with trigger "after the first attack/hit of my ally on this particular enemy"?

Answer 1

This would work in both scenarios

The Ready action states:

[...] First, you decide what perceivable circumstance will trigger your reaction. Then, you choose the action you will take in response to that trigger [...]

You can choose any perceivable circumstance so this would include something such as "When my ally makes an attack", note also that the Ready action states:

[...] When the trigger occurs, you can either take your reaction right after the trigger finishes or ignore the trigger. [...]

Thus if your trigger was "after my ally's attack hits or misses" you could have your Help action affect the second attack your ally makes against an enemy and not the first.

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With eldritch blast things a bit more complicated but the following posts explain that the blasts of this spell strick one-by-one (not simultaneously):

• Do multiple hits of Repelling Blast happen simultaneously or in succession?
• When are targets picked for spells with multiple "missiles"?

Because these blasts are not simultaneous, you can have a Readied action occur between the first and second blasts with a trigger such as "after a blast hits or misses the enemy"

The fact that a Readied action can occur in-between the attacks of eldritch blast is potentially further supported by the fact that a bonus action can occur between these blasts. That said, bonus actions do explicitly state that you can choose when to use them, but I would argue that Readied actions do as well since you choose the trigger after which they occur.

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Keep in mind the limitations of the Help action

The Help action states:

[...] Alternatively, you can aid a friendly creature in attacking a creature within 5 feet of you. [...]

This means that when you take this action (when it is actually triggered) your target must be within 5 feet of you. This could easily go wrong such as if the target moves away before you take the Help action (though you would likely be able to make an opportunity attack in this case).

Answer 2

Yes to scenario 1

The Help action says

Alternatively, you can aid a friendly creature in attacking a creature within 5 feet of you. You feint, distract the target, or in some other way team up to make your ally’s Attack more effective. If your ally attacks the target before your next turn, the first Attack roll is made with advantage.

The Ready action says

First, you decide what perceivable circumstance will trigger your Reaction. Then, you choose the action you will take in response to that trigger, or you choose to move up to your speed in response to it. Examples include “If the Cultist steps on the trapdoor, I’ll pull the lever that opens it,” and “If the Goblin steps next to me, I move away.”

You can certainly perceive an enemy getting attacked, and there is no reason why readying an Help action wouldn’t work.

Also yes to scenario 2

Both Help and Ready are actions that familiars are allowed to take, so the second scenario works mostly the same as the first, except that now you’re casting a spell instead of taking multiple attacks. Eldritch Blast is instantaneous, but with spells involving multiple attacks apparently these are not simultaneous according to this Q&A, so there’s no reason why reactions couldn’t take place between multiple blasts.

But it might not work as well as you hope

Because you only actually take the action when the trigger occurs, the creature you’re helping to attack needs to be within 5 feet of you after the first attack hits. If the creature you’re trying to help attack gets a turn in which it decides to move, or is moved through some other effect, you won’t be able to use your readied Help and might’ve wasted your reaction, so while it may make sense to do this for your familiar it very rarely makes sense for a PC to do this unless you have specific features that enhance Help like the Mastermind.