If a warlock with the Repelling Blast invocation casts Eldritch Blast, is the 2nd attack resolved before or after the target is pushed by the 1st hit?

Question

My question is inspired by this other question:
Does Repelling Blast work once per spell or once per beam?

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A 5th-level Warlock with the Repelling Blast eldritch invocation casts eldritch blast, firing 2 beams at the same target. The first one hits, pushing the target back 10 feet. Does the second attack roll come before or after the target is pushed away?

Two scenarios (both came up in play):

1. The opponent is a ghost who is 5 feet away from a wall. The first blast pushes the ghost through the wall. If the second attack is before the repulsion, both beams hit. If it is after, the wall would block the second beam.
2. The opponent is an ordinary creature that is 5 feet away from the caster. The first roll was taken with disadvantage. If the second attack is before the repulsion, then that attack is also with disadvantage. If it is after, then the caster is no longer within 5 feet of an enemy, so the second attack is not at disadvantage.

Answer 1

The second attack happens after the first is fully resolved.

Rules designer Jeremy Crawford addressed this question in an unofficial tweet from June 2015:

Eldritch Blast: are the attacks resolved in parallel or sequence? Do you have to pick all the targets first before rolling?

Multiple attacks on the same turn aren't simultaneous, unless a feature or spell says otherwise.

So in your two scenarios:

1. If the first attack pushed the target out of line of sight, the remainder wouldn't be able to target it. Scenario one would only allow a single attack on the ghost.
2. Scenario two would have disadvantage on the first shot, and regular attacks on the follow-ups.

This is actually in keeping with melee combat rules as well. Consider that if you choose to shove a target 5 feet away, you won't be able to hit it with a follow-up attack unless you have a reach weapon or additional movement.

Answer 2

The attacks are resolved one at a time

Each beam of eldritch blast involves a separate attack roll and is resolved separately

This is clear from the spell description:

The spell creates more than one beam when you reach higher levels: [...] Make a separate attack roll for each beam.

Thus, we can refer to the section on "Making An Attack":

Whether you're striking with a melee weapon, firing a weapon at range, or making an attack roll as part of a spell, an attack has a simple structure.

1. Choose a target. Pick a target within your attack's range: a creature, an object, or a location.

2. Determine modifiers. The DM determines whether the target has cover and whether you have advantage or disadvantage against the target. In addition, spells, special abilities, and other effects can apply penalties or bonuses to your attack roll.

3. Resolve the attack. You make the attack roll. On a hit, you roll damage, unless the particular attack has rules that specify otherwise. Some attacks cause special effects in addition to or instead of damage.

Thus, we can see that each beam is resolved separately, and in succession; this is further shown by looking at a spell that explicitly calls out its beams being simultaneous, magic missile:

[...] The darts all strike simultaneously [...]

The eldritch blast spell has no similar line, and as such, we conclude that the beams do not strike simultaneously, and thus are resolved one at a time; including their targeting and anything else.

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This is further corroborated by the Sage Advice Compendium under the "Spellcasting" section (sub-section Spell Attacks):

When casting a spell that affects multiple targets, such as scorching ray or eldritch blast, do I fire one ray or beam, determine the result, and fire again? Or do I have to choose all the targets before making any attack rolls?

Even though the duration of each of these spells is instantaneous, you choose the targets and resolve the attacks consecutively, not all at once. If you want, you can declare all your targets before making any attacks, but you would still roll separately for each attack (and damage, if appropriate).

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Using Repelling Blast is up to the caster and happens on a hit

The Invocation states (emphasis mine):

When you hit a creature with eldritch blast, you can push the creature up to 10 feet away from you in a straight line.

From this we can see that using Repelling Blast is a choice (the Invocation uses the word "can") and that choice is made when you hit with eldritch blast. We also know that each beam (and thus each hit/miss) is resolved separately and thus have everything we need to address your two scenarios:

1. If the first blast pushes the Ghost through the wall, the second blast will miss as it cannot target the Ghost through a wall; however, the first blast does not need to push the Ghost at all. The caster can instead choose not to push the Ghost and continue to fire beams of eldritch blast at it and choose to push it after any hit. (Notably, you cannot hit with the first beam, miss with the second, and then choose to push the Ghost through the wall)

2. Say the first attack hits; the caster can now choose to push the creature away (which they likely want to do to avoid having disadvantage on the later beams). Thus they would hit with the first beam, push the creature away (thus removing future disadvantage), and then roll normally for all remaining beams of eldritch blast.

Answer 3

The repulsion happens after the first attack

First, let’s look at when the attacks happen. According to the Sage Advice Compendium (p. 13):

When casting a spell that affects multiple targets, such as scorching ray or eldritch blast, do I fire one ray or beam, determine the result, and fire again? Or do I have to choose all the targets before making any attack rolls?

Even though the duration of each of these spells is instantaneous, you choose the targets and resolve the attacks consecutively, not all at once. If you want, you can declare all your targets before making any attacks, but you would still roll separately for each attack (and damage, if appropriate).

This tells us that the attacks, at least, don’t happen at the same time. The SAC doesn’t say anything about Repelling Blast, so we have to look at what the feature says. The description of the Repelling Blast eldritch invocation says:

When you hit a creature with eldritch blast, you can push the creature up to 10 feet away from you in a straight line.

This doesn’t change when the attacks happen, so the attacks still happen consecutively.

The books never define “consecutive”, so we look at the dictionary definition of the word. The Merriam-Webster dictionary defines consecutive as “following one after the other in order”. So the attacks follow, one after the other.

Repelling Blast activates on a hit (see the text above), and in general, things that trigger on a hit directly follow the hit. Therefore, the second attack roll happens after the target is pushed away (following the first attack roll).

Timeline

1. Targets are declared. The warlock declares they want to use Repelling Blast and target the creature twice.
2. The warlock makes the attack roll for the first attack. It is a hit.
3. The target is pushed back 10 feet.
4. The warlock makes the second attack roll.

After this point, everything continues as normal.

The ghost

In the case of the ghost in front of the wall, the second attack would not hit. The ghost’s Incorporeal Movement trait says “The ghost can move through other creatures and objects as if they were difficult terrain. It takes 5 (1d10) force damage if it ends its turn inside an object”.

This feature says that the ghost can move through objects, not that it must. This leads to the question “Does the ghost even enter the wall?” The SAC says nothing about this, and I can’t find anything on this.

If the ghost does enter the wall, it is pushed back and the second attack will miss because the ghost will have cover. If the ghost does not enter the wall, the second attack could hit because the ghost would not be repelled through the wall.