Do all of the melee spell attacks from Steel Wind Strike have advantage if the caster is under the effect of the Invisibility spell?

Question

The Invisibility spell (PHB, p. 254) says:

A creature you touch becomes invisible until the spell ends. Anything the target is wearing or carrying is invisible as long as it is on the target's person. The spell ends for a target that attacks or casts a spell.

And the spell Steel Wind Strike (XGE, p. 166) says:

You flourish the weapon used in the casting and then vanish to strike like the wind. Choose up to five creatures you can see within range. Make a melee spell attack against each target. On a hit, a target takes 6d10 force damage.

If a spellcaster under the effect of an Invisibility spell casts Steel Wind Strike, would they have advantage on all five melee spell attacks, or only on the first one?

Answer 1

Please note that this answer assumes that steel wind strike does not naturally grant advantage on all its attacks (even without invisibility). I believe this is the intended interpretation, but it may not be the RAW reading -- see this question.

Either the first one, or none of them.

Take eldritch blast as precedent, as the wording is similar. Eldritch blast calls for ranged spell attacks against multiple targets, specifically making "a separate attack roll for each beam" (PHB p.237), similar enough to steel wind strike's "make a melee spell attack against each target."

The Sage Advice Compendium reads as follows, in response to a question asking whether the beams from eldritch blast are simultaneous:

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).

SA-C p.12

Thus, since each attack happens in series, and invisibility ends either when you attack or cast a spell, there are two distinct points when the invisibility could end. If "casting a spell" is considered to occur when you begin casting a spell, then you become visible before any attacks are made. Otherwise, it will last until the first attack breaks invisibility, and that one attack will be made with advantage.

So the issue rests on when the character is considered to have cast a spell. I'm not aware of an official ruling that would make this determination, but if one exists, it will be the determining factor.

Answer 2

You have advantage on every attack.

You would have advantage on all of the attacks, but not because of the effect of invisibility.

First, we understand that there is no flavor text in spell descriptions.

That said, the following sentence is rules, not flavor:

You flourish the weapon used in the casting and then vanish to strike like the wind.

You vanish. If you vanish, you are unseen. If you are unseen, then:

When a creature can't see you, you have advantage on attack rolls against it.

You are an unseen attacker for each attack of steel wind strike, so they would be made with advantage.